[fluxvars] introduce enabled/disabled specific flux specializations

Merged Dennis Gläser requested to merge feature/implement-fluxvars-specializations into master

We now avoid compilation of the calls of the type Law::flux() whenever the process is deactivated. This has to be done because for deactivated processes, the corresponding flux var caches are empty. Compilation of the laws thus fail because they expect their corresponding caches.

Merge request reports