Added option to output structures as standard multi-xyz files

[RaphaelRobidas]

Some molecular viewers cannot read the molden output files produced by pyGSM. I added an option to instead create multi-xyz files containing the relative energies as comments (which are sometimes parsed automatically).

The suggested code works, but you might want to refactor the functions in utilities.manage_xyz; I didn't want to mess too much with your other functions that write XYZ files, so I added a new one that takes the same parameters as write_molden_geoms.

[craldaz]

Hey, this is good. Thanks! I like the dictionary trick you use to import. I unfortunately had a bad habit of importing everything.