Added fdtd-2024 to the stack (external)

Added the latest release of FDTD.

One major change relative to the latest version we had (as a module, at least). There is no more fdtd-engine-mpich-nem as there used to. Right now there are fdtd-engine-ompi-lcl and fdtd-engine-impi-lcl.

This means that from now on the user will need to either module load gcc openmpi fdtd or module load intel intel-oneapi-mpi fdtd. Since we have fdtd in an external section with a cloud switch, I opted to keep things as they are in our files.

Would you rather have some sort of external_mpi_codes codes with the relevant matrices?