BUG: Laminar model not available for buoyantSimpleFoam

Validation case NaturalConvection/2D-Laminar/2DLaminarBuoyantCavity fails.

1. constant/thermophysicalProperties.mixture.equationOfState now needs T0 instead of Tref

2. keyword simulationType is not respected. It looks for RAS only.

@Mattijs @Sergio