DerivationFrameworkHiggs: enable thread-checker and fixes

Remove mutable variables (except counters) from HIGG1, HIGG2, and HIGG5VBF in favor of passing the relevant quantities explicitly.

📢 I'd appreciate a careful review by the shifter.

P.S.: We don't run derivations multi-threaded, but if we decide to do so at some point these kind of changes would have to be done anyway.