LoadVesuvio can now load the sum of foil in and out states

It previously could only load either in or out and not the combination
of both. Refs #6860
Minor refactor of LoadVesiuio for easier access to foil map.

A user can now use the SpectraFoilMap to access what periods are
defined to be in each state. Refs #6860
Merge remote-tracking branch 'origin/feature/6860_loadvesuvio_both_foils'