create rhoeps field in createFields.H

this was a major issue:
rhoeps must be created outside the run loop and have old time values,
otherwise any time derivative will be zero, leading to incorrect results

applications/solvers/cfdemSolverRhoPimple/cfdemSolverRhoPimple.C

@@ -122,7 +122,6 @@ int main(int argc, char *argv[])
             #include "rhoEqn.H"
         }
 
-        volScalarField rhoeps("rhoeps",rho*voidfraction);
         // --- Pressure-velocity PIMPLE corrector loop
         while (pimple.loop())
         {

applications/solvers/cfdemSolverRhoPimple/createFields.H

@@ -65,6 +65,9 @@ Info<< "Reading thermophysical properties\n" << endl;
         dimensionedScalar("zero", dimensionSet(1,-1,-3,0,0,0,0), 0.0)
     );
 
+    volScalarField rhoeps("rhoeps", rho*voidfraction);
+    rhoeps.oldTime(); // switch on saving old time
+
     Info<< "\nCreating fluid-particle heat flux field\n" << endl;
     volScalarField Qsource
     (