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
DataFlash : EKF optical flow fusion support preliminary changes
AP_NavEKF : preliminary implementation for optical flow and range finder fusion

Range finder measurements are not input to EKF at this time, however the method for fusing them is
implemented.