Expose valid kwargs in documentation for .to_xarray() method

[ahuang11]

Following the example in #137

I was trying to do this:
col.search(model='CFSv2', variable='prate').to_xarray(decode_times=False)

But I got the error:

ValueError: some chunks keys are not dimensions on this object: {'time'}

When I tried looking at the docstring, I couldn't find anything that mentioned how to change the time coordinate.

Signature: cat.to_xarray(**kwargs)
Docstring:
Return dataset as an xarray dataset
Additional keyword arguments are passed through to methods in aggregate.py

I had to look at source.py to see the available kwargs:

        if 'decode_times' not in _kwargs:
            _kwargs.update(decode_times=True)
        if 'compat' not in _kwargs:
            _kwargs.update(compat='no_conflicts')
        if 'time_coord_name' not in _kwargs:
            _kwargs.update(time_coord_name='time')
        if 'ensemble_dim_name' not in _kwargs:
            _kwargs.update(ensemble_dim_name='member_id')
        if 'chunks' not in _kwargs:
            _kwargs.update(chunks={_kwargs['time_coord_name']: 'auto'})
        if 'override_coords' not in _kwargs:
            _kwargs.update(override_coords=False)
        if 'join' not in _kwargs:
            _kwargs.update(join='outer')
        if 'preprocess' not in _kwargs:
            _kwargs.update(preprocess=None)
        if 'merge_exp' not in _kwargs:
            _kwargs.update(merge_exp=True)

And finally, I got it to work:
col.search(model='CFSv2', variable='prate').to_xarray(decode_times=False, time_coord_name='target')

So I guess it'd be easier for people if the to_xarray method showed something like (or more detailed):

Signature: cat.to_xarray(**kwargs)
Docstring:
Return dataset as an xarray dataset
Valid kwargs: decode_times, compat, ... etc
Additional keyword arguments are passed through to methods in aggregate.py

[andersy005]

So I guess it'd be easier for people if the to_xarray method showed something like (or more detailed):

I concur

The aggregate.py module will be removed in the next release because most of the things we used to do ourselves have been implemented in xarray>=0.13.

Here's what the to_xarray() is going to look like:

https://github.com/NCAR/intake-esm/blob/87c94eb126f27a4acd897c1ba9789325a8892c4e/intake_esm/core.py#L105-L112

I will update the docstring with your suggestion

[andersy005]

This has been addressed in #135