Fix several missing things in markings modules
Fix several missing things in markings modules

 * show_subframe had "aspect='equal'". Not required as the whole plot is
already manually resized at the end.
 * using add_patch instead of add_artist and adding plot_center back in.
 * fans were not yet converted to Fan objects early on.
 * fans did not yet have a plot_center method, now added.

michaelaye/planet4,michaelaye/planet4,michaelaye/planet4,michaelaye/planet4