wrapping MPI_Wtime with PCU_time

This is part of an effort to have
PCU.h not pull in mpi.h.
this decoupling get us closer
to mpi-free builds, and contain
the area affected by mpi.h,
which in some cases contains warnings.
Merge branch 'master' of https://github.com/SCOREC/core
one more MPI_Wtime conversion