Allow `get_mol_kegg()` to work with pathway modules

[Aariq]

Messing around with KEGG and volcalc and I realized that get_mol_kegg would work with pathway modules in addition to entire pathways except that it errors if the input doesn't start with map (modules look like M00113, for example). https://www.genome.jp/module/M00113

Just need to:

• [ ] Explore what other IDs the API accepts
• [ ] Modify the input validation in get_mol_kegg()
• [ ] Update documentation accordingly