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
Copter: log compass failur to dataflash
Merge branch 'master' of https://github.com/diydrones/ardupilot
Copter: restore logging of arm/disarm event
Copter: remove unused event definitions
Copter: update firmware version and release notes ahead of -rc2
Copter: flexible CH6 declination tuning range