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
373653203,MDU6SXNzdWUzNzM2NTMyMDM=,2508,groupby fails on generic ndarray functions,11671536,open,0,,,4,2018-10-24T20:00:22Z,2020-10-04T16:05:58Z,,NONE,,,,"This seems related to #326.

#### Code Sample, a copy-pastable example if possible

```python
import numpy as np
import xarray as xr

da = xr.DataArray(np.random.randint(0, 1, (10, 10, 3)),  dims=['row', 'col', 'time'])

da.groupby('time').apply(np.linalg.norm)
```
#### Problem description

I would expect xarary to know how to apply generic numpy functions along specified axis. However, it currently raises the following exception:

```python

AttributeError                            Traceback (most recent call last)
<ipython-input-22-5451ed1f09ee> in <module>()
----> 1 da.groupby('time').apply(np.linalg.norm)

~/anaconda3/lib/python3.7/site-packages/xarray/core/groupby.py in apply(self, func, shortcut, **kwargs)
    514         applied = (maybe_wrap_array(arr, func(arr, **kwargs))
    515                    for arr in grouped)
--> 516         return self._combine(applied, shortcut=shortcut)
    517 
    518     def _combine(self, applied, shortcut=False):

~/anaconda3/lib/python3.7/site-packages/xarray/core/groupby.py in _combine(self, applied, shortcut)
    519         """"""Recombine the applied objects like the original.""""""
    520         applied_example, applied = peek_at(applied)
--> 521         coord, dim, positions = self._infer_concat_args(applied_example)
    522         if shortcut:
    523             combined = self._concat_shortcut(applied, dim, positions)

~/anaconda3/lib/python3.7/site-packages/xarray/core/groupby.py in _infer_concat_args(self, applied_example)
    289 
    290     def _infer_concat_args(self, applied_example):
--> 291         if self._group_dim in applied_example.dims:
    292             coord = self._group
    293             positions = self._group_indices

AttributeError: 'numpy.float64' object has no attribute 'dims'


```

#### Expected Output

a data array whit a time coordinate of size 3 (ie same shape as ``da.groupby('time').mean()``)

#### Output of ``xr.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.7.0.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-138-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

xarray: 0.10.9
pandas: 0.23.4
numpy: 1.15.1
scipy: 1.1.0
netCDF4: 1.4.1
h5netcdf: 0.6.2
h5py: 2.8.0
Nio: None
zarr: None
cftime: 1.0.1
PseudonetCDF: None
rasterio: None
iris: None
bottleneck: 1.2.1
cyordereddict: None
dask: 0.19.1
distributed: 1.23.1
matplotlib: 2.2.3
cartopy: None
seaborn: 0.9.0
setuptools: 40.2.0
pip: 18.1
conda: 4.5.11
pytest: 3.8.0
IPython: 6.5.0
sphinx: 1.7.9


</details>
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231715344,MDU6SXNzdWUyMzE3MTUzNDQ=,1428,changes made to coords using groupby and apply do not persist,11671536,open,0,,,1,2017-05-26T19:28:06Z,2020-03-29T15:28:53Z,,NONE,,,,"I am running Ubuntu 16 with Xarray 0.9.1 on python 3.6.0.

I have found that any changes made to coordinates in a function that is called by a groupby object's apply method do not persist. The following code illustrates the problem:

```python
import numpy as np
import xarray as xr


def change_new_coord(dar):
    """""" change the new_coord coord from 1 to 0 """"""
    dar.coords['new_coord'] = 0
    return dar


# setup data array
data = np.ones((10, 10, 1000))
time = np.linspace(0, 10, 1000)
coords = {'time': time, 'd2': range(10), 'd3': range(10)}
dims = ['d2', 'd3', 'time']
dar = xr.DataArray(data, coords=coords, dims=dims)

# attach coordinate based on d2 and d3
dar.coords['new_coord'] = (('d2', 'd3'), np.ones((10, 10)))

# stack
stacked = dar.stack(z=('d2', 'd3'))

# groupby
gr = stacked.groupby('z')

# apply
out = gr.apply(change_new_coord).unstack('z')

# raises; all values in new_coord should be 0, but they are still 1
assert np.all(out.coords['new_coord'] == 0)
```","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/1428/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,issue