Useful functions not in the Array API Standard

[TomNicholas]

There are a few numpy functions which xarray calls on wrapped arrays but which are not (yet) in the Array API Standard. (See https://github.com/data-apis/array-api/issues/187#issuecomment-1553615779) Cubed could choose to implement these to facilitate full integration.

FYI of this list xarray currently uses:

np.clip
np.diff
np.pad
np.repeat
np.take
np.tile

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Of particular interest to me personally is np.pad. It's used within xarray's .pad method, which is used within xGCM's apply_as_grid_ufunc, which led to the pad function being an important part of the test case that exposed memory management problems with dask's distributed scheduler. I can't really close the loop by trying out cubed on that full original problem unless pad is available in cubed.

pad is also interesting because a parallel implementation isn't trivial - dask's pad implementation uses map_blocks in some cases, but more complicated tricks in other cases. For my purposes above I wouldn't need to implement more than one or two of the mode kwarg options though.

I would be interested in submitting a PR for adding pad if that's something you would welcome @tomwhite? (I mentioned this on the xarray tracker but it's really a cubed question https://github.com/pydata/xarray/issues/7848#issuecomment-1553614542)

cc @jbusecke