Adds some unit tests for the solvation module.

This includes tests for: setUp, obtaining solute parameters from a library,
calculating and setting the McGowan volume for a solute, calculating solute
diffusivity, and calculating solvent viscosity.

The main method was also edited so that the unittest can be run on its own.

Along the way fixed a floating point error in calculating diffusivity.
Solvation module raises a DatabaseError when solvent is
not found in library. Unittests updated with a solvent library
test.
Cleaned up tests for Abraham parameter estimation and solvation
correction calculation.

Nitrogen and sulfur cases commented out for now, fixing is work in progress. Removed many test cases, so only have a few for each test.