Understand that multiplicity of '{2S,2T}' is equivalent to '2'.

This is when reading adjacency lists, and makes it more compatible with RMG-Py.
However, I think I'm about to make the database exporter export it as '2' anyway,
so this may not be necessary.
This is a belt-and-braces approach to solve @GreenGroup/RMG-database#18
Understand thermo trees that start at level L1 instead of L0.

I am not sure why the thermo trees used to start with L0
whereas the kinetics trees all begin with L1. In RMG-Py they
all begin at L1, which made exported things incompatible.
Now we try L0, if that fails try L1, and if that fails throw 
an exception.