Saturation values above 1

[paveltomin]

GEOS formulation allows saturation values to go above 1 (not sure about below zero). The computed saturation values are used as is to compute rel perms that might be not really physical thing to do and also may mislead the solver blocking some flow or allowing too much flow. More reasonable way would be to normalize saturation values used for rel perm computes, see, e.g., https://drive.google.com/open?id=10dHD58pO8dy5hSz4RjVo8m0Mjs6C0a69