mesh_loc(y0, x0[, y, x[, ireg]])
mesh_loc returns the zone index (= i+ imax*(j-1)) of the zone of the mesh (x, y) (with optional
region number array ireg) containing the point (x0, y0). If (x0, y0) lies outside the mesh, returns 0.
For example, ireg(mesh_loc(x0,y0,y,x,ireg)) is the region number of the region containing
(x0, y0). If no mesh specified, uses default. x0 and y0 may be arrays as long as they are
conformable.