Add new support for bool/flag user settings, with corresponding processing
Add new support for bool/flag user settings, with corresponding processing by the Pythia8_i core class. Used these to add a new GravFlat:FlatPt flag to the GravFlat user hook, to enable flat pT sampling at 13 TeV as supplied by Francesco Di Bello at https://its.cern.ch/jira/browse/ATLMCPROD-6561