[rotorcraft] guidance_h speed ref saturation fix

Bound ref to max accel along reference vector.
If angle can't be computed, simply set both axes to max magnitude instead of zero.

Should (tm) fix #716
[rotorcraft] gh_max_speed|accel
[conf] remove INS_SONAR_VARIANCE_THRESHOLD
[doc] add basic Python styleguide
[conf] flightplan cleanup
[rotorcraft] add some functions to set heading via flight plan
[rotorcraft] change DEFAULT_CIRCLE_RADIUS to 5m
Merge pull request #717 from paparazzi/fix_speed_ref_saturation

[rotorcraft] guidance_h speed ref saturation fix