Allow it to read reactions with up to 5 products in reaction libraries!

These reactions are a bit fishy to say the least, but some of the chemkin
files we have imported have 5 products, and so we make kinetics.py files
that until this commit, we couldn't then read.
Allow undeclared duplicates if one is pressure-dependent and one is not.

This is how Chemkin behaves, and thus how some of our imported kinetic models look.
The whole treatment of duplicate reactions in RMG needs a serious overhaul,
but this allows the importer tool to understand models it has imported.