mixing VISC4 along s-surfaces to see if that helps with high N2 runs - runew-34
removed mix_geo_ts4 for dx500 ru with no tnu4
reverted to sane mixing rotation + chagned to WJ_GRADP
visc along s-surfaces + no visc_grid + no diff_grid
added back N2S2_HORAVG, DIFF_GRID, VISC_GRID for (hopefully) final compile