AMBER 20 for stage 2023 using gpsmkl

This was previously not possible because mpi4py was not available
with gpsmkl. However, after my fixes concerning Python 3.10 API
changes, the bundled mpi4py is able to compile as well. So, we
can provide AMBER also for gpsmkl. The additional benefit is that
we can provide PLUMED support, as some AMBER users on our computers
have requested it before. PLUMED is installed for gpsmkl, but
not currently for foss.