Introduce Geantinos in the Athena
Add Geantino PDG codes 998/999 to the AtlasPID.h
Both geantinos are strongly interacting. One is neutral and one is positively charged.
Note: MCUtils contain a bug that considers both geantinos to be uncharged mesons, therefore one has to take that into account and adjust the reference files.
Tag @jchapman @pclark @abuckley @ewelina
Fixes ATLASSIM-6612
Edited by John Derek Chapman