Rename motionvideo() to motion() and average_image() to average()

[balintlaczko]

The other video renderer functions you call like: mg.history() --> which makes a 'something_history.avi' if your mg object had 'something.avi' mg.motionhistory() --> 'something_motionhistory.avi' however: mg.motionvideo() --> 'something_motion.avi' :((( mg.average_image() --> 'something_average.png' :(((

[balintlaczko]

Done!