Add additional isomorphism tests to molecular mapping.

[SamTov]

What feature would you like to see added?
We could add two new approximate isomorphism tests to the molecular graph.

• Check total number of bonds is correct. This is the sum over the adjacency matrix.
• Check rank of each atom. This is the row-wise sum over the adjacency matrix.

This will give us the four fundamental tests if isomorphism as well as a species included test giving us an even stronger assurance of isomorphism without a complete test.