indexing with a list of slices

[keewis]

Following the discussion in #10479, this adds support for a list-of-slices container (tentatively called MultipleSlices) to most of the indexing wrapper classes. Still missing: PandasIndexingWrapper.

It didn't look like using a separate class as suggested in #10479 (comment) would be easy, and so I've decided to instead extend _index_indexer_1d to support, which turned out surprisingly easy.

I'll still have to figure out how to best test this for the indexing wrapper classes other than LazilyIndexingWrapper, and I'm not really content with the API and the code duplication I have right now. I'll refactor that soon, but for now this works:

In [5]: import xarray as xr
   ...: import numpy as np
   ...: 
   ...: var = xr.DataArray(np.arange(100), dims="x")
   ...: slice_ = xr.core.indexing.MultipleSlices(
   ...:     slice(7, 12), slice(15, 32, 2), slice(50, None, 3)
   ...: )
   ...: var.isel(x=slice_)
Out[5]: 
<xarray.DataArray (x: 31)> Size: 248B
array([ 7,  8,  9, 10, 11, 15, 17, 19, 21, 23, 25, 27, 29, 31, 50, 53, 56,
       59, 62, 65, 68, 71, 74, 77, 80, 83, 86, 89, 92, 95, 98])
Dimensions without coordinates: x

cc @dcherian

• Closes indexing by a list of slices #10479
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst