Simulators for two-phase gas/water models may report only SWAT. In this case no SGAS result entry was created, so SGAS was missing from the result list and TERNARY fell back to computing SOIL = 1 - SWAT, presenting the gas saturation as oil saturation.
Add RigSgasResultCalculator that creates a placeholder SGAS entry when the model has gas and water but no oil phase, SWAT is present and SGAS is missing. SGAS is then computed as SGAS = 1 - SWAT. The computation is dispatched from findOrLoadKnownScalarResult() and findOrLoadKnownScalarResultForTimeStep() the same way as SOIL.
The equivalent logic in RigCaseCellResultsData::testAndComputeSgasForTimeStep() was only reachable from RigSoilResultCalculator, which never runs for models without an oil phase. This function is replaced by computeSgasForTimeStep() delegating to the new calculator, and the unreachable call in RigSoilResultCalculator is removed.
