No functional change

Move velocity store out of optical flow to velocity and position fusion control as it is a velocity fusion function.
Always clear the previous mode status

Name change done to avoid confusion with flight modes. modes are now referred to as constant position and constant velocity mode.

This patch makes the reporting of an absolute position solution less abbiguaous and ensures that relative position is always true if absolute position is true

Filter is declared unhealthy and will not arm for first 10 seconds after initialisation

Because synthetic  position measurements at 0,0 are fused during position hold mode, a position timeout cannot be used as the only means of detecting position solution status.

This status will be cleared when data arrives and is fused successfully

Required to prevent acquisition of GPS mid flight causing unwanted change in position and velocity
A distinction has been mad between the arm and disarm transition and the decision to use position hold mode (formerly static mode)

200 msec was too short and could lead to false positives. 5000 msec is the largest time we can go free inertial.

This renames static mode as posHoldMode to make it clearer what this and the other non-GPS modes do

When regaining GPS after a timeout, an offset is applied when fusing  GPS velocity so that GPS velocity and position data as fused by the EKF is kinematically consistent.
This velocity offset is also accounted for when fusing air data so that wind estimates are not corrupted when the GPS position offset is being pulle back to zero.
The intended behaviour is that the EKF position will be pulled back to the GPS position at a rate of 5m/s for planes and 1 m/s for copters. This avoids large deviations in trajectory when GPS is regained.

When on the ground it is likely the flow sensor will be returning  data that does not meet the minimum quality requirements selected.
The previous check was for the presence of valid data. This has been loosened to look for the presence of data.
When the vehicle becomes airborne, the quality of flow data normally improves as the image comes into focus.

This patch enables indoor operation of the EKF by putting it into static mode if optical flow operation is not enabled and no GPS is available