GPU aware MPI

[kevinstratford]

There is no portable way to identify, at run time, whether a GPU aware MPI is present.

At the moment we have the compile time switch

#ifdef HAVE_OPENMPI_
/* This provides MPIX_CUDA_AWARE_SUPPORT .. */
#include "mpi-ext.h"
#endif

(e.g., field.c) which is relevant for OPENMPI where the non-standard mpi-ext.h is available.

It is desirable to isolate this rather than have a proliferation of conditional compilation. This might live with the pe_t, e.g.,

pe_have_gpu_aware_mpi()

This may be separate from whether the user wasnts to use it or not.