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.
Understand frequency trees that start at level L1 instead of L0.

Same story as previous commit.
Exporting things from the RMG-database project starts trees at level L1.
For some reason, frequency trees were expected to start at L0.
Now either will work.