Introduced header for communication to/from PME ranks
No functionality changes. This cleans up some structure, and will be
useful for some modernization, use of std::vector, and then new
allocation strategies to suit PME on GPUs.
Eliminated some things in pme-internal.h by moving some declarations
to a header that can be included by the only two source files that are
interested in PP-PME communication. Now gmx_pmeonly() doesn't have to
pass around a large pile of arguments.
Removed a use of typedef struct, and some function parameter types
that no longer need to specify struct in C++.
Removed some unused PP_PME_* constants.
Change-Id: I51629fb6d91b3a486ef24d1f60065e65261d0376