flow: restructure optical flow control logic (#928)

- extract motion checks performed on ground
- move all non-timing check to controlOpticalFlowFusion. This simplifies and standardirzes the setOpticalFlowData function. Furthermore, in some cases (SITL, PX4Flow), the dt is forced to 0 when the quality is 0 (which is usual on ground) so the ekf needs to ignore those values on ground to initialize properly the flow fusion.
- add filter convergence test
- move check for dt time max in setOpticalFlowData
- in the simulator, do not update dt as this is the sensor integration
period.

test/sensor_simulator/sensor_simulator.cpp

@@ -12,8 +12,8 @@ _rng(ekf),
 _vio(ekf),
 _airspeed(ekf)
 {
-	setSensorDataToDefault();
 	setSensorRateToDefault();
+	setSensorDataToDefault();
 	startBasicSensor();
 }