Deal with converters

[IAlibay]

Currently converters are partially dealt with under the umbrella of topology & coordinate parsers. However they are not completely dealt with properly, for example neither RDKit or OpenMM are covered under gen_topologyparser_attrs.

At the very minimum we should clean this up. Ideally we should look to see if we should just separate out converters and deal with them in a separate way than standard topology/coordinate readers?

[orbeckst]

Converters deserve their own section.