Add a function for updating the multiplicity of a molecule.

Calculates the multiplicity using the formula 2s+1 where
s is the unpaired total radical electron count.
Update the multiplicity of a newly formed productStructure when generating reactions.

Was running into problems because the default multiplicity is set to -187, which means
that isomorphism checks checking multiplicity now fail and can declare a product
to be a new species even if it already exists in the model.