Fix H2CC electronic state in USC-Mech-ii thermo database.

We'd attributed it to the triplet, based on the reactions,
but this is the thermo for the singlet. 
(We should have an inter-state crossing reaction in our models)
importOldDatabase.getUsername() now works on posix systems without os.getlogin()

This solves a problem when running on one of our clusters, which would 
otherwise give an OSError when you try to call os.getlogin()
Tweak a comment in R_Addition_MultipleBond/training

When this is exported to RMG-Java style database, it became ambiguous.
(It said "THIS is the full rate not the per-site rate" but that was 
no longer true, because the comment was re-used on a rate-rule which
has been translated into a per-site basis.)
exportOldDatabase.py removes heteroatoms from thermo libraries.

Some thermo libraries contain nitrogen atoms, which confuses RMG-Java.
This prunes those species, so the exported database can run in RMG-Java.
Currently it only allows C,H,O and S through.
Script for converting RMG-Java individual kinetics libraries to Py format.