Rename map_blocks tasks.

[dcherian]

This makes it clearer which tasks are subsetting xarray objects, vs.
actually running the user provided function.

• Tests added
• Passes pre-commit run --all-files
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

import dask

import xarray as xr

da = xr.DataArray(
    dask.array.zeros((10,), chunks=(3,)),
    dims="dim_0",
    coords={"a": (("dim_0",), np.arange(10))},
    name="data"
)


def func(a):
    return a.dim_0


da.map_blocks(func).data.visualize()

before

after

[dcherian]

Note users have been confused by our previous choice of names: https://stackoverflow.com/questions/65320657/resample-and-groupby-on-big-dask-array-with-xarray-using-map-blocks

Final version:

[dcherian]

"this-array" is misleading but attempting to fix it breaks many tests...

[dcherian]

merging in a couple of days if no one has any comments.