Copter: add pitch axis flipping

Added Pitch axis flipping. You must be pushing forward or back 3° to make it flip in Pitch axis or it will flip in the default roll axis.
Added some extra throttle control to maintain altitude
removed the CH7 ability to exit Flip mode. It will exit only if completing flip or user aborts with control sticks.
Copter: flip minor comments and formatting
Copter: remove unused flip_stop
Copter: ensure motors don't stop during flip