Implement shapesys modifiers in config

[alexander-held]

Add support for shapesys modifiers via a new type of systematic for cabinetry. They would behave somewhat similarly to NormPlusShape, but are independent per bin, would not have a norm/shape split, and would come with Poisson constraint instead of Gaussian constraint. The implementation could probably closely follow NormPlusShape.

Discussed in #453

^{Originally posted by Tomoya-Iizawa January 18, 2024}
Hello,

I would like to ask if there is any way to implement systematics with decorrelated among bins?
For example, if we have bins [0, 250, 500, 750, 1000] and 2-point shape systematics as ttbar-ME, is it possible to decorrelate by each bin?