Update CScalarSolver.inl

Added comment on use of current density

Author: emaberman

SU2_CFD/include/solvers/CScalarSolver.inl

@@ -503,6 +503,8 @@ void CScalarSolver<VariableType>::CompleteImplicitIteration(CGeometry* geometry,
       SU2_OMP_FOR_STAT(omp_chunk_size)
       for (unsigned long iPoint = 0; iPoint < nPointDomain; iPoint++) {
         /*--- Multiply the Solution var with density to get the conservative transported quantity, if necessary. ---*/
+        /* Note that for consistency with residual and jacobian calaulcations, use of current density for conservative variables 
+        * of the old solution is used. see pull request https://github.com/su2code/SU2/pull/2458*/
         const su2double density = flowNodes->GetDensity(iPoint);
 
         for (unsigned short iVar = 0; iVar < nVar; iVar++) {