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
Merge branch 'master' of git://github.com/diydrones/ardupilot into Branch_7f624e68a176f7adc3e2955837878a3c6dc2c40d

# By Andrew Tridgell (58) and others
# Via Andrew Tridgell
* 'master' of git://github.com/diydrones/ardupilot: (80 commits)
  SITL: fixed simulated airspeed value
  AP_HAL: removed scaling factor on analog sources
  Copter: fixed RSSI reading on PX4
  Plane: removed scaling of pitot source
  Plane: use voltage_average() for RSSI
  Rover: use voltage_average() for RSSI
  AP_ADC_AnalogSource: added voltage_average_ratiometric()
  AP_Rangefinder: use ratiometric analog input for analog sonar
  AP_Airspeed: use rationmetric analog input for airspeed sensor
  AP_HAL: added voltage_average_ratiometric() call to AnalogIn
  PX4: fixed handling of nostart option
  AP_AHRS: use vectors for ground vector complimentary filter
  AP_L1_Control: Remove potential nan errors
  Copter: bug fix for slow loiter repositioning
  PX4: don't start px4io twice
  MAVLink: update to latest common.xml from upstream
  Copter: bug fix for RTL bearing
  WPNav: make get_stopping_point method public
  Compass: update parameter description
  Copter: allow lat/lon to be specified for land
  ...