FluxVarsCache: Check if throat is invaded to calculate WettingLayerArea

What this MR does / why does DuMux need it:

I see the following issue in the update function of the fluxvarscache:

• if a theta function is present in the problem, it is not checked if the throat is invaded to calculate the wettingLayerArea. If the throat is not invaded a WettingLayerArea is calculated based on the curvatureRadius, contactAngle and cornerHalfAngles, which is lower than the total Area. This leads also to a positive throat cross-sectional area for the non wetting phase.

This merge request inserts a check if the throat is invaded to calculate the WettingLayerArea if a theta-function is used in the problem.

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