Introduce a new saturation endpoint parameter to replace krgendanchor

[berland]

Similar to how sgro is used to control the upper left portion of the GasOil curves, a similar saturation endpoint should control the upper right part instead of the krgendanchor string argument, which creates additional code paths. Since the situation is analoguous, the implementation should be analoguous.

The krgendanchor argument is probably very seldomly used, but backwards compatibility should be possible.