Add examples of loading GROMACS topologies mixed with data from `$GMXDATA`

[mattwthompson]

          TOP files produced by GROMACS typically include ITP files distributed with GROMACS stored in `${GMXDATA}/top`. It might be worth attempting to load files from there if no local file can be found, eg something like:

                try:
                    included_file = open(itp_file)
                except FileNotFoundError as e:
                    included_file = open(f"{sys.env.get("GMXDATA", "")}/{itp_file}")
                with included_file as itp_obj:

I think in practice this won't actually allow many more TOP files to be loaded as they also mostly need #IFDEF...#ENDIF handling to deal with position restraints, but it'd be good future proofing.

Originally posted by @Yoshanuikabundi in #1120 (comment)