Automatically create xindex

[ianhi]

Closes: #9703

allows .sel to automatically create a PandasIndex object for if one is not present.

import xarray as xr
ds = xr.Dataset(
    {"data":('time', [0,1,2,3])},
    coords={"time":[.1, .2, .3, .4], 'time_metadata':('time', [10,15,20,25])}
)
ds
# both these work and are equivalent
ds.sel(time=.2)
ds.sel(time_metadata=15)

• Closes #xxxx
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• [NA] New functions/methods are listed in api.rst

@max-sixty @benbovy

[benbovy]

Great! Thanks @ianhi.

[max-sixty]

super, thanks @ianhi !