Address the speed of plotting large hggroup groups and in particular contours (bug #31305). Changes to address this include - Use __go_patch__ in __contour__ rather than patch so that the cost of setting up the callback functions is avoided. The contourgroup callback handles the updating of properties. - Add children_property class to store children in a list so that adding and deleting children is a low cost operation. - Create a new version of update_axis_limits code that doesn't force the recalculation of all of the objects children. Patch also allows unclosed patch contours with the FLTK backend.