PG-PuReMD: fix issue with certain data structures not being reallocated when...

PG-PuReMD: fix issue with certain data structures not being reallocated when the local number of atoms owned by a processor increases after exchanging messages with neighbor processors. Fix an issue where the MPI send message buffer may be overwritten by a received message (Coll). Small fix to not overwrite the program status return value with local function return values. Other code clean-up.