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

Continuing to remove multiplicity from Species attributes, and making it part 
of the Molecule or Group, represented via AdjacencyLists.
Correct "special cases" for carbon monoxide SMILES strings.

Not sure why we need a special case to read carbon monoxide (perhaps
this can be removed), but if we do, then we ought to be honest,
and not read the SMILES for hydroxymethylene as if it is CO.

Overall, I think CO chemistry needs to be looked at carefully.