This release adds gradient estimates and patches some bugs

• The turtle_map_gradient and turtle_stack_gradient provide gradient estimates w.r.t. map coordinates.
• Bugs have been patched in turtle_stepper concerning local transforms for maps.
• .asc files are now assumed to be in reading order.
• .png files now use full numeric precision for metadata.
• A turtle_stepper_reset function was added in order to manually reset the stepping history.