Thermodynamic property target

This pull request adds the thermodynamic property target by Erik Brandt.  This target enables the optimization of any combination of thermodynamic properties, and eventually we should be able to replace liquid.py with this more general approach.  Each new thermodynamic property is coded as an instance of the "Quantity" class.

I also added a liquid bromine test case (identical to the "liquid" target). 

I'll leave this open for ~3 days, then I'll merge it into the master.