Use cumsum from flox

[Illviljan]

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

The non-flox version reduces chunksizes significantly:

x = xr.DataArray([1, 1, 1, 1, 1], name="x").chunk()
grp_idx = xr.DataArray([-1, 0, 0, -1, 1])
with xr.set_options(use_flox=False):
    print(x.groupby(grp_idx).cumsum())
<xarray.DataArray 'x' (dim_0: 5)> Size: 40B
dask.array<getitem, shape=(5,), dtype=int64, chunksize=(2,), chunktype=numpy.ndarray>
Dimensions without coordinates: dim_0

With flox the chunksize is retained:

x = xr.DataArray([1, 1, 1, 1, 1], name="x").chunk()
grp_idx = xr.DataArray([-1, 0, 0, -1, 1])
with xr.set_options(use_flox=True):
    print(x.groupby(grp_idx).cumsum())
<xarray.DataArray 'x' (dim_0: 5)> Size: 40B
dask.array<_finalize_scan, shape=(5,), dtype=int64, chunksize=(5,), chunktype=numpy.ndarray>
Dimensions without coordinates: dim_0

[dcherian]

yes, this is the way. eventually I'd like the apply_ufunc for reductions to live in Xarray too. So feel free to move that over if it helps. We could put it in flox_compat.py

[dcherian]

probably requires a version guard.

[Illviljan]

Ahh, now I see:
groupby_scan was added in: https://github.com/xarray-contrib/flox/releases/tag/v0.9.9 - OK
cumsum was added in: https://github.com/xarray-contrib/flox/releases/tag/v0.10.5 - NOK