MPI_Aint addressing arithmetic functions needed

[hppritcha]

The MPI-3.1 MPI_Aint helper functions MPI_Aint_add and MPI_Aint_diff need to be implemented.
See MPI-Forum ticket 349 and sections 2.6.4 and 4.1.5 of the MPI-3.1 standard document.

[jsquyres]

This is now done.