Copter: sanity check throttle deadzone
AP_NavEKF : Reduce ripple in estimates that can cause copter motor 'pulsing'

This patch reduces the level of 5Hz and 10Hz 'pulsing' heard in motors due to GPS and altimeter fusion which cause a small 5Hz and 10Hz ripple on the output under some conditions. Attitude, velocity and position state corrections from GPS, altimeter and magnetometer measurements are applied incrementally in the interval from receiving the measurement to the predicted time of receipt of the next measurement. Averaging of attitude state corrections is not performed during periods of rapid rotation.

Time stamps are now explicitly initialised to the current IMU time to avoid unwanted activation of timeout logic on filter start and the various calls to the hal.scheduler->millis() object have been consolidated.

Unused variables have been removed
Build: enable px4flow driver on Pixhawk
OptFlow: rename and restructure OpticalFlow class

AP_OpticalFlow.h becomes simply a file that includes all other optical
flow header files.
OpticalFlow class simplified to only return surface quality, raw output
and velocity vector.
OptFlow: reorganise ADNS3080 to simplified interface
OptFlow: add PX4Flow support
PX4: start px4flow driver
Copter: enable OPTFLOW - do not merge to master
Copter: optflow parameters moved to optflow class
Copter: simpler optflow logging

Removed of_loiter control output from message
Copter: instantiate optflow on Pixhawk

Run sensor reads from scheduler
Copter: integrate optflow_enable param move to optflow class
Copter: Of_Loiter uses sensor velocity instead of integrated position
Copter: OF log fix to save vel as float
Copter: add TimeMS to OF dataflash log msg
OptFlow: add ground_distance_m
OptFlowPX4: retrieve ground distance from sensor
Copter: log ground distance from optflow sensor
Copter : EKF optical flow fusion support preliminary changes

Assumes optflow sensor has been modified to push data at a low rate, eg 10Hz
and velocity data is angular velocity, not linear velocity
AP_AHRS : EKF optical flow fusion support preliminary changes