Fix sequential flash:
- correct and more general calculation of equilibrium constants (divided by wrong phase pressure before) => small changes compared to reference
- generalize 2-phase check by using phase mass fraction instead of feed mass fraction (first component had to be main component of first phase before this change) => does not change solution compared to reference
- cleanup, comment, add literature
This was done/checked for the 1p2c flash and the 2p2c saturation flash, too, and the reference solutions were corrected.