interFoam: add support for p-relaxation

applications/solvers/multiphase/interFoam/pEqn.H

@@ -49,6 +49,8 @@
         {
             phi = phiHbyA - p_rghEqn.flux();
 
+            p_rgh.relax();
+
             U = HbyA + rAU*fvc::reconstruct((phig - p_rghEqn.flux())/rAUf);
             U.correctBoundaryConditions();
             fvOptions.correct(U);