Wrapped mpif.h in module mpif to avoid include statements.

This is mostly in response to gcc's -check all -Wall -Wextra throwing
a fit every time mpif.h was included (because the majority of the
include file wasn't being used, and it needed to tell me that).
The module mpif is simple enough that it doesn't need to be
checked, and it can be `use mpi, only : ` to check for variable
usage and have a name declaration at the top of every file.