Representation decorator needs a base_type

[cgreenberg]

Currently the base type is assumed to be IMP.atom.BALLS, but that shouldn't be required. Maybe we want to represent a fragment with only density.

Also, any objections if I just replace everywhere IMP.atom.BALLS to IMP.atom.BEADS ?

[benmwebb]

Sounds reasonable, but you can't just do a wholesale search and replace - we'd need to keep BALLS as a (deprecated) synonym to avoid breaking any existing code that uses it (probably not much).