mpi: use mpi get_accumulate as proxy for atomic add

[vmiheer]

Partly fixes #99. I think for other functions ideally the imples for atomic should be deleted giving compile time error message to the user of krs/mpi. But atleast it should print warnings saying the library is using non-atomic operations for now.

Get_accumulate might be slightly inefficient than just Accumulate but c++ opeartor += should return the updated value, maybe it could return "optional" value, the access to which would error out.

[janciesko]

Superseded by https://github.com/kokkos/kokkos-remote-spaces/pull/103

[vmiheer]

Closing.