Introducing liquid thermo libraries in RMG

As discussed with @rwest, this small changes introduce a new kind of libraries usefull for liquid phase part.
I tried to write it as clean as possible with a lot of information in commented lines but please tell me which part seems poorly written to not reproduce my errors again.

There is also a bug fix related to new changes in process thermodata done in 80e9759 and 7b25cf5  which modified NASA polynomial coefficients for species matching thermo library (gas or liquid phase). 

I tried many cases locally, for me it looks safe.

nb: I want also to create the same kind of library for the kinetic that's why I implemented the solvent attribute like this. The keyword can already be read by RMG but all the other parts need to be done (like do not apply solvation reaction matching liquid reaction library, ...)