AMBER 20 with AmberTools 21 for foss toolchain

A lot of patches were necessary in the sources, in C, C++
Python and CMake, but we have a working version of AMBER-20
with the other packages in the 2023 stage. The most trouble
came from certain API changes in Python 3.10. It is easy to
compile AMBER 20, but because of changes to the interfaces
of some Python--C interface functions like Py_REFCNT, which
were used in a lot of places in AMBER, over 500 changes
spread over 25 files are necessary. In addition, in this
version, we make sure that fftw and netcdf are used from
the installed versions instead of using the bundled version
inside AMBER.