Refactored path planner. The path planner's role in navigation is to take
user-defined waypoints and provide a path that connects them together.

The path planner is supplied with two algorithms, "direct" and "direct-
with-filleting".

"Direct" goes directly to the waypoint, no matter how severe the
angle between two consecutive waypoints.

"Direct-with-filleting" adds fillets
to the direction changes, in order to provide smoother, more realistic
paths for dubins cart vehicle classes, i.e. vehicles that have a minimum
turn radius.
Added circular moudulus for radians.
Refactored fixed-wing path follower to use math from known good version.
Also added new follower, based on freshly released work from R. Beard's
lab.
Added UAVOs to Navigation category.
Added support for new navigation chain to FlyingF3 target. An option was
provided to easily disable the path planner.