InputConverter: Refactoring
Rename methods which create G4PrimaryParticles for pre-defined decays so that their purpose is clearer.
Move code used to process the pre-defined decays of quasi-stable particles
from getG4PrimaryParticle
method into a separate processPredefinedDecays
method.
Make the linking of G4PrimaryParticles in pre-defined decay-chains optional.
(Preparatory changes for !63059 (merged).)