Proper implementation of cubic equations of state

[g-bauer]

With support for

• Setting the global parameters to define a generalized cubic ($\delta_1$, $\delta_2$)
• Generalized $\alpha$ functions and some default implementations for commonly used ones (Soave, Twu, Mathias-Copeman)
• Generalized mixing rules

Todos

• Adjust algorithms (density iteration, critical points) for cubics
• more mixing rules (including gE models)
• add volume translation