FastCaloSim: Making FastShowerCellBuilderTool const.
TH1::GetRandom() and TH2::GetRandom2() are not MT-safe, since they're non-const and also use gRandom. These are used from ParticleEnergyParametrization. Rewrite so as to be const and to allow passing in a random generator rather than having to use gRandom. (We currently still pass in gRandom. Changing that would change rng seeding, so that will be fixed in a subsequent change.)