Update State class

[ibell]

The State class has a few problems when using the HEOS backend with mixtures in PDSim. For one, the phase flag is never able to be used due to a bytes <-> str type mismatch in the comparison and also, the AbstractState class being managed is not accessible, and some methods are only available in the AbstractState class (for instance first_partial_deriv). So the pointer has been exposed, and the phase is handled (more) properly now.