Add --fast option to MFEM build script

[Heinrich-BR]

The update_and_rebuild_mfem.sh script nominally contains the --fast option. In principle, passing this flag will cause the script to skip the configure step of building MFEM and go straight to make. However, as the script is currently structured, passing the --fast flag does nothing, as the script will still configure and make MFEM normally.

It would be ideal to adapt the update_and_rebuild_mfem.sh script so that it could make use of the --fast option.