forgot to do a branch for this. I sorted out some gridding questions, particularly fixed the triangulation object made in readgrid.py to be on a psi grid instead of a rho grid, so that, for example, a drifter in the middle of a cell has the grid-relative position of [something].5, [something].5. I have updated the tests, and made sure that inner walls as well as open numerical boundaries are behaving as expected. Results have been written in document on this.