HistWrapper depends on MAT-MINERvA via useNuEConstraint and Flux-related stuff

[bamesserly]

This is a dependence of MAT on MAT-MINERvA. It means without MAT-MINERvA one cannot use HistWrappers.

Compilation is fine, but it's a runtime error when using HistWrappers.

The dependence is "just" a sanity check / catch on the number of flux universes being considered. I don't have a current plan for how to move this into MAT-MINERVA. First thought would be to spin off a MAT-MINERvA child of HistWrapper, which is probably not ideal. Better to move it elsewhere; maybe somehow into MinervaUniverse?