check fingerprint, multiplicity in Molecule.findIsomorphism(...)

This leads to code duplication with regards to Molecule.isIsomorphic(...)
However, a call to Molecule.isIsomorphic(...) inside
Molecule.findIsomorphism(...) would lead to twice the execution of the
isomorphism algorithm, rather than just once.
added 4 tests for multiplicity in molecule isomorphism
hundt's rule detecting u2 in combination with mult = 1

added some tests showing the behavior.