AP_NavEKF: Fix bug preventing the non-GPS mode being selected externally

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.
AP_NavEKF: remove unnecessary function
AP_NavEKF: Fix error in comments
AP_NavEKF: Add static mode to solution status message
AP_NavEKF: Improve behaviour recovering from a GPS timeout

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.
AP_NavEKF: Compensate optical flow fusion for GPS glitch recovery offset
AP_NavEKF: Rename static mode to avoid confusion with other non-GPS modes

This renames static mode as posHoldMode to make it clearer what this and the other non-GPS modes do
AP_NavEKF: Use compensation for baro delay  in position hold mode
AP_NavEKF: Relax timeout check applied to optical flow data

200 msec was too short and could lead to false positives. 5000 msec is the largest time we can go free inertial.
AP_NavEKF: Latch use of position hold mode for duration of flight

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)
AP_NavEKF: Initialise timeout status to true

This status will be cleared when data arrives and is fused successfully
AP_NavEKF: Link process noise to arm status, not pos hold mode
AP_NavEKF: Fix bug in reporting of solution status

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.
AP_NavEKF: Allow 10 seconds for the filter to settle after initialisation

Filter is declared unhealthy and will not arm for first 10 seconds after initialisation
AP_NavEKF: Modify initial values to make startup behaviour more consistent
AP_NavEKF: Ensure initialisation methods read required sensor data
AP_NavEKF: make position and velocity resets consistent with GPS usage
AP_NavEKF: Ensure filter doesn't try to use GPS after initialisation
AP_NavEKF: clean up determination of GPS availability