Allow `__getitem__` indices that have a `to_dask_array` method

[davidhassell]

It would be useful to allow cf.Data and other objects (such as cf.DimensionCoordinate) to be used as indices within cf-python.

E.g. so we could do a northern hemisphere subspace like this:

>>> g = f[f.dimension_coordinate('Y') > 0, ...]

Note: you can already do this with g = f.subspace(Y=cf.gt(0)), but there's no reason to preculde the [] approach, which currently does not work (but did pre 3.14.0).

PR to follow.

Thanks to @sws06mas for bring this to my attention.