Change TransportData __repr__ function and fix pdep/network#_#.py files.

@enochd pointed out in a comment to commit 
8a6d32441007649807e3137ba7da0e6c2dab9f6d
that RMG-py was creating cantherm/measure input files
(eg. pdep/network23_1.py) that were malformed.

This should fix it, I hope.
Fix bug in TransportData.__repr__

Small error in 3c0a68e413d362f307593cfd1ec3339b402914b8 
was turning 'comment' into """'comment'""".
Added Style guidance to developer documentation.
getCollisionFrequency unittest fix

the previous test was using the wrong numeric value for the collison
frequency at the specified conditions. The test will pass now.
Set default option partCoreEdge='core' in saveOutputFile function to not break old code.
Return a proper Chemkin species identifier even if it exceeds 15 characters.

15+ character species identifiers work fine in later versions of CHEMKIN.
It might break CHEMKIN 2.0 though, but let's just print a warning for it.
Write a useful error message when you cannot save a species to the dictionary when writing chemkin file.