Set MPI parallelization scheme by environment variable
Until now, the MPI parallelization scheme used in a NIFTy installation is hard coded at installation time. With this patch, the MPI parallelization scheme can be set via the environment variable NIFTy_MPI_SCHEME at runtime.
Possible values are "Standard", "Samples" and "None".
If the environment variable is not set, the "Standard" scheme is chosen.