This commit introduces a set of consistency checks for the gas phase
saturation functions. These plug into the framework introduced in
commit c3939c544 (PR #5438). We implement the following three checks
- 0 <= SGL < 1
- 0 <= SGU < 1
- SGL <= SGCR < SGU
which collectively enable a non-negative oil saturation in the two
phase gas/oil system.
