metabolite support is incomplete and not implemented properly

HMDB was added, but no other chem sources.
https://github.com/cytoscape/idmapper/blob/master/src/main/java/org/cytoscape/idmapper/MappingSource.java#L9

1. Should add other common chem sources
2. Should restrict "to" list to same type as "from" list, e.g., chem types or gene types, since mapping from HMDB to Ensembl (or vice versa) makes no sense and will always fail.

Start with src/main/java/org/cytoscape/idmapper/MappingSource.java and trace how its identifier sources populate the available from and to lists. Determine which common chemical sources are required and how source types are represented; done means those sources are supported and incompatible mappings, such as HMDB to Ensembl, cannot be selected.

Written by the indexing model from the issue text.