Implement NumPy's __array_function__ protocol for array methods that are not in the Array API Standard

[tomwhite]

This is to support nanmean, nansum (see #153), and pad (#461).

The previous attempt at this in #468 was reverted in #469, since it was causing problems when used with Xarray (pydata/xarray#8834 (comment)).

When Xarray is working we can re-introduce the changes here.

[tomwhite]

The Xarray change that enables this has been merged (pydata/xarray#9530).

I've re-created the branch from #468 here: https://github.com/cubed-dev/cubed/tree/np-array-function. However, when I run the cubed-xarray tests I get some failures:

FAILED cubed_xarray/tests/test_wrapping.py::test_to_zarr[single-threaded] - TypeError: no implementation found for 'numpy.isclose' on types that implement __array_function__: [<class 'cubed.array_api.a...
FAILED cubed_xarray/tests/test_wrapping.py::test_to_zarr[processes] - TypeError: no implementation found for 'numpy.isclose' on types that implement __array_function__: [<class 'cubed.array_api.a...
FAILED cubed_xarray/tests/test_xarray.py::TestVariable::test_binary_op_bitshift - TypeError: no implementation found for 'numpy.full_like' on types that implement __array_function__: [<class 'cubed.array_api...
FAILED cubed_xarray/tests/test_xarray.py::TestVariable::test_reduce - TypeError: no implementation found for 'numpy.isclose' on types that implement __array_function__: [<class 'cubed.array_api.a...
FAILED cubed_xarray/tests/test_xarray.py::TestDataArrayAndDataset::test_lazy_array - TypeError: no implementation found for 'numpy.isclose' on types that implement __array_function__: [<class 'cubed.array_api.a...
FAILED cubed_xarray/tests/test_xarray.py::TestDataArrayAndDataset::test_dataarray_pickle - TypeError: no implementation found for 'numpy.full_like' on types that implement __array_function__: [<class 'cubed.array_api...
FAILED cubed_xarray/tests/test_xarray.py::TestDataArrayAndDataset::test_dataset_pickle - AssertionError: Left and right Dataset objects are not identical
FAILED cubed_xarray/tests/test_xarray.py::test_unify_chunks - AssertionError: Left and right Dataset objects are not identical
================================ 8 failed, 41 passed, 2 skipped, 15 xfailed, 3 warnings in 3.47s =================================

There is no isclose function in the Array API (data-apis/array-api#170), so we may need to build one (in Xarray?).

cc @keewis @TomNicholas