Provide a shorthand for "one bin along this axis" in .rebin

[Cadair]

Describe the feature

If you want to "drop" an axis by averaging along an axis then you need to know the length of that axis to do rebin((nx, 540, 20)) it would be nice to provide a shorthand for this, I propose a -1 sentinel so for a cube with dimensions (100, 100, 100) .rebin((-1, -1, 50)) would give you a cube with shape (1, 1, 2).

Proposed solution

No response

[DanRyanIrish]

I support this. And it should be simple to implement. It may make sense to implement this after #726 is merged, as it moves the code where this needs to be implemented.