Users should control whether their slice_by_gate_type outputs are split into depth-1 layers

[caleb-johnson]

If we do not split things by color in our collect function, we automatically split out slices of a gate type into depth-1 layers. This can have an impact on how the budget is divided and may not always be desirable. Surface this Qiskit arg as a kwarg to our slicing functions to allow users to have a little more control over this.

I would go as far as to say I think the default behavior should be that they are not broken out into single-depth layers, but I don't feel that strongly about it.