Only write multiplicity in adjacency lists, and store on Molecules.

@connie and @rwest have decided...

* Store multiplicity on Molecule objects.
* Ensure every molecule in a Species has the same multiplicity
* Move the "does the multiplicity make sense?" check to Molecule.fromAdjacencyList()
* Always write multiplicities in Adjacency Lists, NOT sometimes as attributes to database entries.

This may not be finished yet, and may need another commit or two to finish off...