ISO_Fortran_binding.h and MPICH

A project switched to Fortran mpi_f08 after realizing runtime crashes in very large simulations were due to MPI-2. The Fortran mpi_f08 interface was released in 2012 and is well supported by contemporary compilers and MPI libraries.

A limitation became apparent with Clang and MPICH, which doesn’t build right now due to Clang not yet having ISO_Fortran_binding.h and Flang being not yet ready for public use. GCC + MPICH or Clang + OpenMPI and many other combinations work. macOS users who desire MPICH with Fortran mpi_f08 bindings need to use GCC and build MPICH themselves.