This subroutine compute the TVD flux function @param q_cons_vf Cell-averaged conservative variables @param q_prim_vf Cell-averaged primitive variables @param rhs_vf Cell-averaged RHS variables @param i Dimensional splitting index Computes viscous terms @param q_cons_vf Cell-averaged conservative variables @param q_prim_vf Cell-averaged primitive variables @param rhs_vf Cell-averaged RHS variables