Support DataArrays with names that are not string

[malmans2]

Right now pretty much everything is broken if any DataArray in a Dataset has a name that is not a string.
However, I don't think we have to introduce too many changes to fix this, it involves something very similar to what we did in #227.

What do you think, does it makes sense to support these kind of Datasets?

[dcherian]

👍, technically variable names can be Hashable, they're typed like that. So we can relax the str typing slowly. I initially had it typed as Hashable but then the errors were too hard to fix :/ so I switched to str in the mean time.

[dcherian]

I think we can do this incrementally by changing the typing pretty low down in apply_mapper and rewrite_values and work up from there.

[aulemahal]

Moving my other issue to here, because it's the same base problem.

The use of ds._to_temp_dataset() is broken with cf_xarray because the resulting data variable doesn't have string name, but rather some xarray ReprObject.

import xarray as xr
import cf_xarray as cfxr

ds = xr.Dataset(
    {'a': (('x',), [1, 2, 3, 4])},
    coords={ 'x': [2, 3, 4, 5]}
)

print(cfxr.__version__)
print(ds.a._to_temp_dataset().cf)

Fails with:

Traceback
  File "/home/aulemahal/test.py", line 14, in <module>
    print(ds.a._to_temp_dataset().cf)
  File "/home/aulemahal/.pyenvs/testcfxr/lib/python3.9/site-packages/cf_xarray/accessor.py", line 1283, in __repr__
    text += make_text_section("Bounds", "bounds", coords)
  File "/home/aulemahal/.pyenvs/testcfxr/lib/python3.9/site-packages/cf_xarray/accessor.py", line 1229, in make_text_section
    vardict = getattr(self, attr, {})
  File "/home/aulemahal/.pyenvs/testcfxr/lib/python3.9/site-packages/cf_xarray/accessor.py", line 1910, in bounds
    vardict = {
  File "/home/aulemahal/.pyenvs/testcfxr/lib/python3.9/site-packages/cf_xarray/accessor.py", line 1912, in <dictcomp>
    apply_mapper(_get_bounds, obj, key, error=False)
  File "/home/aulemahal/.pyenvs/testcfxr/lib/python3.9/site-packages/cf_xarray/accessor.py", line 93, in apply_mapper
    raise ValueError("`default` must be provided when `key` is not a string.")
ValueError: `default` must be provided when `key` is not a string.

Because key is an instance of xr.core.utils.ReprObject.

[dcherian]

Yes lets fix this. A general solution would be nice, but using isinstance(ReprObject) would be an OK quick fix. It must be the most common non-string name.