Added endmember Gibbs with Landau, and a new test script.

Conflicts:
	burnman/minerals/HP_2011.py
Started work on Bragg-Williams order-disorder.
Continued work on Bragg-Williams order-disorder.

Conflicts:
	burnman/minerals/HP_2011.py
Attempts to locate tc-burnman discrepancies
Implemented HP2011 Landau and Bragg-Williams order-disorder models for endmembers
Added comment to warn that Cp, V where Landau/Bragg-Williams order-disorder is implemented are currently incorrect
Minor edit
Implemented sample_pressure attribute for experimental data inversion
Moved HP2011 test to tests directory
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