Prune votes for things with big enthalpy discrepancies.

Currently set at 150 kJ/mol
Perhaps should depend on how good other matches are.
Use pruned votes to decide if a match is unique.

Before it would use pruned votes for one-to-many check, 
and all votes for many-to-one. Now it's pruned in both directions.
Tiny tweaks for speed.

May not even help, but this is the slowest function
Better help when making manual matches
Speed up the reaction comparison function.

Reduce namespace lookups, and len() calls, and comparisons.

It turns out
    len(a)==1 and len(b)==1
is twice as fast as
    len(a)==len(b)==1

Should probably port these changes into the main RMG-Py algorithm.
Make all SMILES output canonical.

The OpenBabel documentation admits this is slower (I've not checked how much)
but the results are much prettier, as well as reproducible from run to run.
(Requires a cython rebuild)
Only print voting when you need user input.