Compass: update parameter description

We do not want people modifying the COMPASS_MOTCT manually
WPNav: make get_stopping_point method public
Copter: bug fix for RTL bearing

Also smooth RTL's initial climb stage by projecting stopping point
MAVLink: update to latest common.xml from upstream
PX4: don't start px4io twice
Copter: bug fix for slow loiter repositioning
AP_L1_Control: Remove potential nan errors

If WP A and B were the same or ground speed was exactly zero, then the
previous code would produce a nan output.  Protection against these
two cases has been added.

If WP A and B are equal, we track directly to the target waypoint
AP_AHRS: use vectors for ground vector complimentary filter
PX4: fixed handling of nostart option
AP_HAL: added voltage_average_ratiometric() call to AnalogIn

this is for ratiometric sensors such as the 3DR airspeed sensor and
the Maxbotix analog sonar
AP_Airspeed: use rationmetric analog input for airspeed sensor
AP_Rangefinder: use ratiometric analog input for analog sonar
AP_ADC_AnalogSource: added voltage_average_ratiometric()
Rover: use voltage_average() for RSSI

this allows it to work on PX4
Plane: use voltage_average() for RSSI

this fixes it on PX4
Plane: removed scaling of pitot source
Copter: fixed RSSI reading on PX4

needed for different analog input scaling
AP_HAL: removed scaling factor on analog sources

these are not use anymore, as voltage_average() is used instead
SITL: fixed simulated airspeed value
Plane: mark ArduPlane 2.74beta