Add custom PhaseDiagram constructor

[prehner]

This allows code like

PhaseDiagram(
    [
        State(eos, temperature, pressure=p, moles=feed).tp_flash()
        for p in pressure
        if not State(eos, temperature, pressure=p, moles=feed).is_stable()
    ]
)

From the resulting PhaseDiagram object, the properties can then be obtained without having to write loops.

Similar to that StateVec is exposed in Python and gets a corresponding constructor, that can be used like this:

StateVec([State(eos, temperature, pressure=p, moles=feed) for p in pressure])

Other smaller changes:

• PhaseDiagram "inherits" the const generic that indicates the number of phases from PhaseEquilibrium. It was a somewhat unnecessary limitation, that could make future additions breaking changes. In Python PhaseDiagram is still fixed to 2 phases and there is currently no analog to ThreePhaseEquilibrium as it isn't needed.
• StateVec::moles is added as an additional getter. A small inconvenience: At this point it is not possible to index SIArray2s in Python.

[g-bauer]

Very nice additions!