Add interface for thermodynamic potentials
Update name of gibbs function
Add einstein thermal model
Use einstein.py to do the heat capacity stuff, implement a few of the not-yet-completed thermodynamic functions
Remove unnecessary inheritance
Bugfixes
Add the ability to calculate Helmholtz and Gibbs free energies to SLB, and more generally to anything using the Debye model.
Remove already inherited methods
Calculate Gibbs and Helmholtz in the mineral class, but fail silently if they are not implemented/unable.
First alpha of SolidSolution base class
Corrected sign of S excess
Added configurational entropy to solidsolution.py
Rename to emphasize extensive quantity
Added more attributes to the solidsolution class
Rejigged SolidSolution class
Added check for endmember and solid solution member compositional equality
Documented and changed name of sample_pressure (now eos_pressure)
Changed comment blob for burnman.SolidSolution
Add pressure to the various EOSs that need it
Initial version of SolutionModel class, including one that does nothing, and IdealSolution class, a SymmetricVanLaar class, and an AsymmetricVanLaar class.  Still work to be done

Conflicts:
	burnman/minerals/HP_2011_ds62.py