Use RDKit instead of Openbabel in toSMILES() for N2 species

fixes #175
Speed up the SMILES generation by looking up small molecules, and test it.

The previous algorithm checked the molecular formula anyway, so we may as well
use that to shortcut a bunch of things that can be uniquely identified that way,
to avoid calling OpenBabel or RDKit.

I also made a unit test that we get back the same SMILES we started with.

Inspired by and closes #175