PG-PuReMD: begin merging preconditioner code from sPuReMD. Make data structures between sPuReMD and PG-PuReMD consistent. Update control file parsing.

PG-PuReMD: merge memory management and step retry logic from MPI+GPU code into MPI-only code. Fix and backport numerous bugs (void pointer arithmetic and dereferences, etc.). Update code to match new data structures. Begin merging changes from OpenMP code (sPuReMD) for preconditioning work.

PG-PuReMD: begin enhancements. Enable sim. step retries. Enable far neighbors list max per atom. Other general code clean-ups.

PG-PuReMD: fix include guards, resolve implicit function declarations, fix inline issues with GCC v5 (C99 std). Other general code clean-up.

PG-PuReMD: fix issue with hydrogen bond cutoff with systems without hydrogen atoms. Remove debugging info for MPI-only version of PG-PuReMD.