Cleaner implementation of the mixed model: tauij = 2 K_sgs / L_kk L_i…

…j - C_mixed * 2 * turb_visc * S_ij:

- the array tauij now contains full mixed model, not just DSTM part
- the passive scalars do not have turbulent diffusion.  The SGS part os modeled by Harlow model.
- only (BT) and (eps T) have turbulent diffusion, because that comes from k-equation.  The k-equation has turbulent diffusion to model the effects of pressure term and triple-correlation term.  Thus the only scalars that should have the turbulent diffusion are (BT) and (eps T), which sum up to k_sgs.