Add a split method?

[nvaytet]

With labeled dimensions, we can do Numpy's split, hsplit, vsplit, and dsplit with a single function.

What should it return? Numpy returns a tuple of arrays.
The function would be similar to sc.collapse (but collapse always reduces the output to 1D arrays), which returns a dict.
A dict is useful for sending to the plotting functions.

[SimonHeybrock]

See also #2873.