ENH: simpleReactingParcelFoam - added density under-relaxation

applications/solvers/lagrangian/reactingParcelFoam/simpleReactingParcelFoam/pEqn.H

@@ -1,6 +1,4 @@
 {
-    rho = thermo.rho();
-
     // Thermodynamic density needs to be updated by psi*d(p) after the
     // pressure solution - done in 2 parts. Part 1:
     thermo.rho() -= psi*p;
@@ -54,6 +52,7 @@
     rho = thermo.rho();
     rho = max(rho, rhoMin);
     rho = min(rho, rhoMax);
+    rho.relax();
 
     Info<< "p min/max = " << min(p).value() << ", " << max(p).value() << endl;
 }