Find and include MPI
h5parm depends on HDF5, which depends on MPI. When building, the mpi.h header file is not found (when it is not installed in a system path). Therefore, we need to explicitly find MPI in CMake and add an include directory.
h5parm depends on HDF5, which depends on MPI. When building, the mpi.h header file is not found (when it is not installed in a system path). Therefore, we need to explicitly find MPI in CMake and add an include directory.