When using optical flow for terrain aiding, we cannot assume that the
terrain estimate is valid if flow is fused as is can only be observed
during motion. When no direct observation is available, the terrain is
assumed to be valid if its 1sigma uncertainty is < 10% of the hagl.
Use flow rates instead of integrals in backend. This allows us to delay
the data to the mitpoint integration time and simplifies the code in
general.
Gyro compensation can still be done in EKF2 if needed, but the
flow module normally already appends the correct gyro data to the flow
message.
