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
324350248,MDU6SXNzdWUzMjQzNTAyNDg=,2159,Concatenate across multiple dimensions with open_mfdataset,35968931,closed,0,,,27,2018-05-18T10:10:49Z,2019-09-16T18:54:39Z,2019-06-25T15:50:33Z,MEMBER,,,,"#### Code Sample

```python
# Create 4 datasets containing sections of contiguous (x,y) data
for i, x in enumerate([1, 3]):
    for j, y in enumerate([10, 40]):
        ds = xr.Dataset({'foo': (('x', 'y'), np.ones((2, 3)))},
                         coords={'x': [x, x+1],
                                 'y': [y, y+10, y+20]})

        ds.to_netcdf('ds.' + str(i) + str(j) + '.nc')

# Try to open them all in one go
ds_read = xr.open_mfdataset('ds.*.nc')
print(ds_read)
```
#### Problem description

Currently ``xr.open_mfdataset`` will detect a single common dimension and concatenate DataSets along that dimension. However a common use case is a set of NetCDF files which have two or more common dimensions that need to be concatenated along simultaneously (for example collecting the output of any large-scale simulation which parallelizes in more than one dimension simultaneously). For the behaviour of ``xr.open_mfdataset`` to be n-dimensional it should automatically recognise and concatenate along all common dimensions.

#### Expected Output
```
<xarray.Dataset>
Dimensions:  (x: 4, y: 6)
Coordinates:
  * x        (x) int64 1 2 3 4
  * y        (y) int64 10 20 30 40 50 60
Data variables:
    foo      (x, y) float64 dask.array<shape=(4, 6), chunksize=(2, 3)>
```

#### Current output of ``xr.open_mfdataset()``
```
<xarray.Dataset>
Dimensions:  (x: 4, y: 12)
Coordinates:
  * x        (x) int64 1 2 3 4
  * y        (y) int64 10 20 30 40 50 60 10 20 30 40 50 60
Data variables:
    foo      (x, y) float64 dask.array<shape=(4, 12), chunksize=(4, 3)>
```
","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/2159/reactions"", ""total_count"": 4, ""+1"": 4, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,completed,13221727,issue
391865060,MDExOlB1bGxSZXF1ZXN0MjM5MjYzOTU5,2616,API for N-dimensional combine,35968931,closed,0,,,25,2018-12-17T19:51:32Z,2019-06-25T16:18:29Z,2019-06-25T15:14:34Z,MEMBER,,0,pydata/xarray/pulls/2616,"Continues the discussion from #2553 about how the API for loading and combining data from multiple datasets should work. (Ultimately part of the solution to #2159)

@shoyer this is for you to see how I envisaged the API would look, based on our discussion in #2553. For now you can ignore all the changes except the ones to the docstrings of `auto_combine` [here](https://github.com/pydata/xarray/compare/master...TomNicholas:feature/nd_combine_new_api?expand=1#diff-876f2fcf4679457325e8018f6d98660cR650), `manual_combine` [here](https://github.com/pydata/xarray/compare/master...TomNicholas:feature/nd_combine_new_api?expand=1#diff-876f2fcf4679457325e8018f6d98660cR550) and `open_mfdataset` [here](https://github.com/pydata/xarray/compare/master...TomNicholas:feature/nd_combine_new_api?expand=1#diff-e58ddc6340e4b5dd0a6e6b443c9a6da1R483).

Feedback from anyone else is also encouraged, as really the point of this is to make the API as clear as possible to someone who hasn't delved into the code behind `auto_combine` and `open_mfdataset`.

It makes sense to first work out the API, then change the internal implementation to match, using the internal functions developed in #2553. Therefore the tasks include:

- [x] Decide on API for 'auto_combine' and 'open_mfdataset'
- [x] Appropriate documentation
- [x] Write internal implementation of `manual_combine`
- [x] Write internal implementation of `auto-combine` 
- [x] Update `open_mfdataset` to match
- [x] Write and reorganise tests
- [x] Automatically ordering of string and datetime coords
- [x] What's new explaining changes
- [x] Make sure `auto_combine` and `manual_combine` appear on the [API page](http://xarray.pydata.org/en/stable/api.html) of the docs
- [x] PEP8 compliance
- [x] Python 3.5 compatibility
- [x] AirSpeedVelocity tests for `auto_combine`
- [x] Finish all TODOs
- [x] Backwards-compatible API to start deprecation cycle
- [x] Add examples from docstrings to main documentation pages","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/2616/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,pull