constants: Standardise use of kmol instead of mol

This prevents spurious factors of 1000 from being introduced in
thermodynamic models. It also generalises the system further with
respect to alternative unit sets.

applications/solvers/chemFoam/pEqn.H

@@ -8,7 +8,7 @@
             invW += Y[i][0]/W[i];
         }
 
-        Rspecific[0] = 1000.0*constant::physicoChemical::R.value()*invW;
+        Rspecific[0] = constant::physicoChemical::RR.value()*invW;
 
         p[0] = rho0*Rspecific[0]*thermo.T()[0];
         rho[0] = rho0;