Ckdtree without cartesian coordinates in grid file

Some grids (e.g. at DWD) do not contain the Cartesian coordinates of the grid's cells, edges and vertices. In a previous version of pyicon, those were derived from the clon, clat, elon, elat, vlon and vlat. However, this was omitted in a performance improvement of the ckdtree_points function. It is possible desirable to recover this option in case that a grid file does contain the cartesian coordinates.