Tweak chemkin thermo file reader.

Make it slightly more honest with regard the Chemkin specification,
and yet slightly more robust for files that break it!

We now read the fifth element/count specifier, if it exists.
Also in this commit, we TRUNCATE ALL SPECIES NAMES TO 18 CHARACTERS.
Remove obvious errors from n-Heptane "CONFIRMED" species list.

How did these duplicates get in here, and some with completely
the wrong chemical formulae?
I'm slightly nervous about the whole file now. How could this happen?
Set singlet[CH2] and triplet[CH2] in n-Heptane model.

We should remember to put this in the instructions for importing a new model!
Added thermo source tooltips to enthalpy discrepancies.

Whenever an H(800K) discrepancy is reported, if you hold the mouse
over it for a few seconds, it now shows you where the thermo came from
(which library, or how it was estimated).

(Apologies to folks using their iPhones, for whom hovering is hard!)