get3_centroid(xyz [, nxyz])
). If NXYZ is not specified, XYZ should be a quadrilateral mesh, ni-by-nj-by-3
(as for the plf function; see page
). In the first case, the return value is len(NXYZ) in length; in
the second case, the return value is (ni-1)-by-(nj-1)-by-3.
The centroids are constructed as the mean value of all vertices of each polygon.