EEqn: Added rho*(U&g) source term

Generally this term has a VERY small effect on temperature, it is only
important for low-speed buoyancy-dominated flows.

Resolves bug-report http://www.openfoam.org/mantisbt/view.php?id=1755

See also http://cfd.direct/openfoam/energy-equation/

applications/solvers/combustion/reactingFoam/EEqn.H

@@ -17,7 +17,8 @@
         )
       - fvm::laplacian(turbulence->alphaEff(), he)
      ==
-        reaction->Sh()
+        rho*(U&g)
+      + reaction->Sh()
       + fvOptions(rho, he)
     );