fixed a bug in readgrid.py that messed up the mapping and therefore kmt. Also altered a check in diffusion.f95 that is supposed to check to make sure the drifter is in the domain, both before and after it has been altered by diffusion if using that flag. The previous check did not appear to make sense to me, so I changed it. I am not sure though why it would have been incorrect. In any case, it seems to run fine now, vertically. I also ran a couple of previous test cases but in 3D and with the different diffusion methods, to compare with 2D and no diffusion, and they all produce qualitatively the same results.