Build micromegas more elegantly

See https://gitlab.cern.ch/jwuerzin/Run3ModelGen/-/blob/main/source/micromegas_5.2.1/CMakeLists.txt?ref_type=heads#L5 and https://gitlab.cern.ch/jwuerzin/Run3ModelGen/-/blob/main/doc/micromegas_build.md?ref_type=heads

Currently, since micromegas is using a large number of difficult to read Makefiles, I didn't manage to get it to build out-of-source. My hacky solution is to copy the entire micromegas directory into the build dir and build "in-source" there.

This should of course be cleaned up.