add interp code for modifiers (generic labels infrastructure?)

[lukasheinrich]

Description

some modifiers implement their modification as an interpolation between user-provided data (e.g. histosys and normsys). As these computations are typically expensive we want to vectorize these as much as possible #231 (even across multiple modifiers -- the meta-modifier idea). To do this we need a way to group these computations by e.g. interpolation type.

comment:
This is not a generic property of modifiers and we are now adding more and more decorators to them (is_constrained, pdf_type, interp_code..) so maybe at some point we might consider a generic labels key-value infrastructure instead of adding properties one by one

[kratsg]

Given the way #285 is going -- this is going to be slowly irrelevant as we've moved the interpolation into the paramset labeling and other properties like is_constrained will be going away -- and overall, the modifier decorator is most likely going to disappear.