array api - Add tests for aggregations

[Illviljan]

• Closes Some aggregate operations using the array API fail #7243
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

[Illviljan]

The array api standard doesn't define any nan*-functions.
xarray pretty much always defaults to using nan*-function. :(

So any time an array has a __array_namespace__ we have to use our own nan-solution, like for .sum:

xarray/xarray/core/duck_array_ops.py

Lines 288 to 295 in d6d2450

	def sum_where(data, axis=None, dtype=None, where=None):
	xp = get_array_namespace(data)
	if where is not None:
	a = where_method(xp.zeros_like(data), where, data)
	else:
	a = data
	result = xp.sum(a, axis=axis, dtype=dtype)
	return result

Any thoughts? Are there any clever ways to handle the aggregations with NaNs in a generic way?

edit: numpy implementations:
https://github.com/numpy/numpy/blob/main/numpy/lib/nanfunctions.py

[tomwhite]

I was hoping https://github.com/data-apis/array-api-compat might help with this, but I'm not sure it does...

[tomwhite]

Related issue: data-apis/array-api#621