CombineTools updates

 * added bbb merging function
 * dropped process_id from ch::Process and replaced with simple bool
   flag to indicate whether process is signal or not
 * made sure functions creating new ch::Nuisance objects also
   add the coresponding paramter if it doesn't already exist
 * In datacard writing, try and deduce the param lines using
   constrained parameters that appear in workspaces, though this
   probably still needs some work (i.e. should get list from the
   present RooAbsPdf objects instead)
 * started adding documentation to CombineHarvester.h