[rotorcraft] refactor guidance_h_ref and fix route_ref if both argumens for atan2 are zero

not really sure what the max_ref values should be in the case where the route_ref angle can't be computed...
set to max on both axes for now...

see also issue #521
[guidance_h_ref] turn some defines into static const
[guidance_h_ref] split up and fix ref max accel/speed calculation
Merge pull request #522 from flixr/guidance_h_ref_fixes

[rotorcraft] refactor guidance_h_ref and fix route_ref