Support shape-only systematic variations

[alexander-held]

As raised in #445: beyond Normalization and NormPlusShape there should be a type for normalized, shape-only variations. Maybe something like ShapeOnly or NormalizedShape. Suggestions welcome!

In some cases a per-channel normalization (which this would implement) is not necessarily the best way to implement variations, as acceptance effects between channels get lost. For these cases, an inclusive normalization that preserves per-channel acceptance effects, but retains the normalization across all channels (or in some other more inclusive phase space) may be better motivated, but would be more challenging to implement.