Airplane code improvements (#180)
* FW: Minimum ground speed guard (applied when POS controller is active)
* FW: Bugfixing
* FW: Min velocity controller will now always add extra throttle above fw_min_throttle
* FW: Increased gain for speed boost
* FW: Cleanups
* FW: Better YAW control for doing coordinated turns in automated modes
* FW: Don't force MAG mode in automated modes
* FW: Improved settings for throttle boost
* FW: Default throttle boost threshold to 7 m/s
* FW: Low-pass filter on roll correction for smoother navigation
* FW: Forced turn direction hysteresis to enforce smooth turn when doing automated 180