Add

1. Export midlevel functions. These are described in the manual, so it seems sensible that they be exported.
2. Switch error message in check  on Z matrix in add-coords to conform to (what I understand to be) the usual convention.
3. Change addcoords and llplot so that surf can plot lists of points, sort of like Mathematica's ListPlot3D: e.g. surf([1:10], [1:10], [1:10].^2, "plotstyle", "points")