Document the minimal supported MPI version

[masterleinad]

Related to #7677. Looking at FindMPI.cmake it doesn't seem feasible to check for the MPI version being at least 2.0 since there seem to be cases where we fail to detect the correct version.

[drwells]

Are we actually compatible with MPI 2.0? I have no idea if we are using any newer features or not.

[masterleinad]

All I can say is that we test for 2.2 in mpi.cc. Hence, I guess that someone tried build with something less than this at least at some point.

[masterleinad]

For compiling with OpenMPI-1.2.9 (MPI-2.0) I needed some changes:

• I could only find MPI_Comm_create_group used in ScaLAPACKMatrix::copy_from and ScaLAPACKMatrix::copy_to in the MPI-3.0 documentation. Hence, these function are declared as not implemented if the detected MPI version is less than 3.0.
• MPI_Isend uses a non-const buffer before MPI-3.0. Hence, we need the DEAL_II_MPI_CONST_CAST macro in a couple more places.

[masterleinad]

/rebuild

[drwells]

Thanks for checking (and digging up this ancient version of openmpi!)