Make dependence on MDTraj and MDAnalysis optional
These two dependencies are not so easy to install and cause problems on a lot of systems. As our Python package gets more users, I would like to make MDTraj and MDAnalyis as optional as possible.
They are currently only used by some of our MDParsers. Particularly Gromacs is making NOMAD fail, because MDAnalysis gets imported when the parser list is compiled in /nomad/parsing/parsers.py
. We need to make sure that imports are optional; either with try/catch or by being imported only on use. Ideally the parsers will react to missing MDAanalysis and disable a few features on parsing with a logged warning.