Adding 'compressible' flag in pEq for chtMultiRegionFoam and buoyantPimpleFoam, rhoThermo and

transient based solvers to account for incompressible Eq of State laws. It avoids taking into account
the term ddt(rho) as mass contribution due to compressibility effects

tutorials/heatTransfer/buoyantPimpleFoam/hotRoom/system/fvSolution

@@ -60,6 +60,8 @@ PIMPLE
     nOuterCorrectors 1;
     nCorrectors     2;
     nNonOrthogonalCorrectors 0;
+    pRefCell        0;
+    pRefValue       1e5;
 }