DecayTreeFitterAlg: Allow for fitting with different mass hypothesis

Feature to allow for decay tree fitting with different mass hypothesis for final state particles. Previously, TupleToolDecayTreeFitter allowed this via Substitution attribute where users pass a dictionary {decay descriptor component: "new-pid"} e.g.

dtt = DecayTreeTuple(..)
dtt.addBranches({'Dstar': '[D*(2010)+ -> (D0 -> K- K+) pi+]CC'})
dtt.Dstar.addTupleTool('TupleToolDecayTreeFitter/ConsDKpi')
dtt.Dstar.ConsDKpi.daughtersToConstrain = ['D0']
#substitue with new pid (Note CC does not work)
dtt.Dstar.ConsDKpi.Substitutions = {
    'Charm -> (D0 -> K- ^K+) Meson': 'pi+',
    'Charm -> (D~0 -> K+ ^K-) Meson': 'pi-'
}

Was thinking of a new property (something like SubstitutePID) for DecayTreeFitterAlg that takes such a map using SubstitutePIDTool as before.

FYI: @pkoppenb and @erodrigu