Docs: MPI Standard

Created by: ax3l

Just because I didn't find it in the docs: what's the minimum MPI standard/API interface you expect? MPI 2.3+?

I was recently installing against OpenMPI 1.6.5 on a legacy CI script (MPI 2.1 on Ubuntu "Trusty") and just saw it didn't pass

adios2/engine/insitumpi/InSituMPIReader.cpp:456:9:
error: no matching function for call to 'MPI_Isend'
note: candidate function not viable: no known conversion from 'const char *' to 'void *' for 1st argument OMPI_DECLSPEC  int MPI_Isend(void *buf, int count, MPI_Datatype datatype, int dest, ...

Of course, there is no reason to support such ancient MPI's but we might want to document and enforce this check within CMake.