Topographic stress

If ltopostress is .true., then an implementation of a topographic stress parameterization [8] is enabled. In effect, this changes the field acted on by the Laplacian operator from (U,V) to (U-U*,V-V*) where (U*,V*) are derived based on the topography gradient and a specified length scale (note that this only works for Laplacian mixing). A smoothed topography can be used to compute this gradient with the number of smoothing passes with a 9-point averaging stencil is governed by nsmooth_topo.