FIX: Thermodynamics when loading databases without mobility parameters

[nury12n]

This fixes a bug that was introduced in 0.3.1 where the GeneralThermodynamics class would assume that a database would always contain mobility parameters and could build a mobility model.

Changes:

• GeneralThermodynamics will check if mobility models were built before creating a PhaseRecordFactory
• State variables used during building of thermodynamic and mobility models will use the GeneralThermodynamics.stateVariables list rather than hard-coding the list several times.
• Unit test added to make sure a database without mobility terms could be loaded