R-hadron cleanup for master

This drags a large number of changes from releases 21.0, 21.3, and 21.6 into master. The RHadron code was largely overhauled. I've assumed that we will never again run Pythia6, so I've removed the Pythia6 functionality from G4ExternalDecayer.

This compiles but does not yet run. It's enough changes and I'm busy enough that I'll put it in now (it's still better than what's there), and let @jchapman take a look...