id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
546303413,MDU6SXNzdWU1NDYzMDM0MTM=,3666,Raise nice error when attempting to concatenate CFTimeIndex & DatetimeIndex,22454970,open,0,,,9,2020-01-07T14:08:03Z,2021-07-08T17:43:58Z,,NONE,,,,"#### MCVE Code Sample
<!-- In order for the maintainers to efficiently understand and prioritize issues, we ask you post a ""Minimal, Complete and Verifiable Example"" (MCVE): http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports -->

```python
import subprocess
import sys
import wget
import glob

def install(package):
    subprocess.check_call([sys.executable, ""-m"", ""pip"", ""install"", package])
try:
    from xclim import ensembles
except:
    install('xclim')
    from xclim import ensembles

outdir = 'tmp'
url = []
url.append('https://github.com/Ouranosinc/xclim/raw/master/tests/testdata/EnsembleStats/BCCAQv2+ANUSPLIN300_ACCESS1-0_historical+rcp45_r1i1p1_1950-2100_tg_mean_YS.nc')
url.append('https://github.com/Ouranosinc/xclim/raw/master/tests/testdata/EnsembleStats/BCCAQv2+ANUSPLIN300_BNU-ESM_historical+rcp45_r1i1p1_1950-2100_tg_mean_YS.nc')
url.append('https://github.com/Ouranosinc/xclim/raw/master/tests/testdata/EnsembleStats/BCCAQv2+ANUSPLIN300_CCSM4_historical+rcp45_r1i1p1_1950-2100_tg_mean_YS.nc')
url.append('https://github.com/Ouranosinc/xclim/raw/master/tests/testdata/EnsembleStats/BCCAQv2+ANUSPLIN300_CCSM4_historical+rcp45_r2i1p1_1950-2100_tg_mean_YS.nc')
for u in url:
    wget.download(u,out=outdir)
datasets = glob.glob(f'{outdir}/*1950*.nc')
ens1 = ensembles.create_ensemble(datasets)
print(ens1)

```

#### Expected Output
Following advice of @dcherian (https://github.com/Ouranosinc/xclim/issues/281#issue-508073942) we have started testing builds of ```xclim``` against the master branch as well as the current release:

Using xarray 0.14.1 via pip the above code generates a concatenated dataset with new added dimension 'realization'

#### Problem Description
 
using xarray@master the ```xclim.ensembles.create_ensemble``` call gives the following error:

```
Traceback (most recent call last):
  File ""/home/travis/.PyCharmCE2019.3/config/scratches/scratch_26.py"", line 23, in <module>
    ens1 = ensembles.create_ensemble(datasets)
  File ""/home/travis/github_xclim/xclim/xclim/ensembles.py"", line 83, in create_ensemble
    data = xr.concat(list1, dim=dim)
  File ""/home/travis/.conda/envs/xclim_dev/lib/python3.7/site-packages/xarray/core/concat.py"", line 135, in concat
    return f(objs, dim, data_vars, coords, compat, positions, fill_value, join)
  File ""/home/travis/.conda/envs/xclim_dev/lib/python3.7/site-packages/xarray/core/concat.py"", line 439, in _dataarray_concat
    join=join,
  File ""/home/travis/.conda/envs/xclim_dev/lib/python3.7/site-packages/xarray/core/concat.py"", line 303, in _dataset_concat
    *datasets, join=join, copy=False, exclude=[dim], fill_value=fill_value
  File ""/home/travis/.conda/envs/xclim_dev/lib/python3.7/site-packages/xarray/core/alignment.py"", line 298, in align
    index = joiner(matching_indexes)
  File ""/home/travis/.conda/envs/xclim_dev/lib/python3.7/site-packages/pandas/core/indexes/base.py"", line 2385, in __or__
    return self.union(other)
  File ""/home/travis/.conda/envs/xclim_dev/lib/python3.7/site-packages/pandas/core/indexes/base.py"", line 2517, in union
    return self._union_incompatible_dtypes(other, sort=sort)
  File ""/home/travis/.conda/envs/xclim_dev/lib/python3.7/site-packages/pandas/core/indexes/base.py"", line 2436, in _union_incompatible_dtypes
    return Index.union(this, other, sort=sort).astype(object, copy=False)
  File ""/home/travis/.conda/envs/xclim_dev/lib/python3.7/site-packages/pandas/core/indexes/base.py"", line 2517, in union
    return self._union_incompatible_dtypes(other, sort=sort)
....
  File ""/home/travis/.conda/envs/xclim_dev/lib/python3.7/site-packages/pandas/core/indexes/base.py"", line 498, in __new__
    return DatetimeIndex(subarr, copy=copy, name=name, **kwargs)
  File ""/home/travis/.conda/envs/xclim_dev/lib/python3.7/site-packages/pandas/core/indexes/datetimes.py"", line 334, in __new__
    int_as_wall_time=True,
  File ""/home/travis/.conda/envs/xclim_dev/lib/python3.7/site-packages/pandas/core/arrays/datetimes.py"", line 446, in _from_sequence
    int_as_wall_time=int_as_wall_time,
  File ""/home/travis/.conda/envs/xclim_dev/lib/python3.7/site-packages/pandas/core/arrays/datetimes.py"", line 1854, in sequence_to_dt64ns
    data, copy = maybe_convert_dtype(data, copy)
  File ""/home/travis/.conda/envs/xclim_dev/lib/python3.7/site-packages/pandas/core/arrays/datetimes.py"", line 2060, in maybe_convert_dtype
    elif is_extension_type(data) and not is_datetime64tz_dtype(data):
  File ""/home/travis/.conda/envs/xclim_dev/lib/python3.7/site-packages/pandas/core/dtypes/common.py"", line 1734, in is_extension_type
    if is_categorical(arr):
  File ""/home/travis/.conda/envs/xclim_dev/lib/python3.7/site-packages/pandas/core/dtypes/common.py"", line 387, in is_categorical
    return isinstance(arr, ABCCategorical) or is_categorical_dtype(arr)
  File ""/home/travis/.conda/envs/xclim_dev/lib/python3.7/site-packages/pandas/core/dtypes/common.py"", line 708, in is_categorical_dtype
    return CategoricalDtype.is_dtype(arr_or_dtype)
  File ""/home/travis/.conda/envs/xclim_dev/lib/python3.7/site-packages/pandas/core/dtypes/base.py"", line 256, in is_dtype
    if isinstance(dtype, (ABCSeries, ABCIndexClass, ABCDataFrame, np.dtype)):
  File ""/home/travis/.conda/envs/xclim_dev/lib/python3.7/site-packages/pandas/core/dtypes/generic.py"", line 9, in _check
    return getattr(inst, attr, ""_typ"") in comp
RecursionError: maximum recursion depth exceeded while calling a Python object
```


#### Output of ``xr.show_versions()``
<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.7.5 (default, Oct 25 2019, 15:51:11) 
[GCC 7.3.0]
python-bits: 64
OS: Linux
OS-release: 4.15.0-74-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_CA.UTF-8
LOCALE: en_CA.UTF-8
libhdf5: 1.10.4
libnetcdf: 4.6.3
xarray: 0.14.1+37.gdb36c5c0
pandas: 0.25.3
numpy: 1.17.4
scipy: 1.3.1
netCDF4: 1.5.3
pydap: None
h5netcdf: 0.7.4
h5py: 2.9.0
Nio: None
zarr: None
cftime: 1.0.4.2
nc_time_axis: None
PseudoNetCDF: None
rasterio: 1.1.1
cfgrib: None
iris: None
bottleneck: 1.3.1
dask: 2.6.0
distributed: 2.6.0
matplotlib: 3.1.1
cartopy: None
seaborn: None
numbagg: None
setuptools: 41.6.0.post20191030
pip: 19.3.1
conda: None
pytest: 5.2.2
IPython: None
sphinx: None

</details>
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