Automatic limit setting for Volume.draw and same scaling for axes

[vischia]

• If the user does not specify xlim, ylim, zlim, they are figured out from the min/max of the volume coordinates (current behaviour: the user is forced to specify the axes limits)
• When the axes limits are automatically determined, force them to have the same scale

[vischia]

May use e.g. self.get_passive_z_range() for this