v1.3.1: Patch for plotting the polygons with 3D bounds

Patch for plotting the polygons with 3D bounds

Changed

• the values 'tri', 'tricontour' and 'tricontourf' for the plot formatoptions have been depreceated and should be replaced by 'poly', 'contour' and 'contourf' respectively, see #23

Fixed

• A bug was fixed with the extend formatoption if plot=None, see #20
• variables with 3D bounds are now interpreted correctly, see #24