Run on Cubed

[tomwhite]

This is an umbrella issue to track the work needed to run sgkit on Cubed.

This is possible because Cubed exposes the Python array API standard as well as common Dask functions and methods like map_blocks and Array.compute. Also, there is ongoing work to Integrate cubed in xarray, as a part of exploring alternative parallel execution frameworks in xarray.

[tomwhite]

I've managed to get some basic aggregation tests in test_aggregation.py passing with the changes here: https://github.com/tomwhite/sgkit/commit/83ff40011b1c985cfca086d3fdf70edb371b3689. This is not to be merged as it's just a demonstration at the moment. Most of the changes are due to the array API being stricter on types (so it needs some explicit casts).

They rely on some changes in xarray too: https://github.com/pydata/xarray/pull/7067.

[tomwhite]

Also, this example shows that Cubed works with Numba (locally at least), which answers @hammer's question here: https://github.com/pystatgen/sgkit/issues/885#issuecomment-1209288596.