Decouple lhcb-gaussino-fastsim

This is needed because we would d like to test Geant4 new cmake with lhcb-gaussino, but it won't work with all the developments related to fast simulation in Gaussino, and in particular, Geant4 10.7. Therefore, the lhcb-gaussino-fastsim label cannot be added in the same MRs as lhcb-gaussino.

I added also a commit that decouples fast simulation developments from Futurev4 by targeting a new branch FASTSIM for Gaussino & Gauss.