[WIP] nglview support other molecular datatypes

[chrisbarnettster]

Easy as pie to support these other datatypes, that's already in the commit

Still to do:

• [ ] Gro support. Gro is set to be a tabular file...
• [ ] fix builtin selections which don't always update for PDB and for small molecules.
• [ ] extend for pv
• [ ] allow for selections - there is an elegant DSL and this function could be brought to the UI
• [ ] allow cumulative representations

[bgruening]

@chrisbarnettster thanks a lot for working on this. Looping in @anuprulez ....