EKF: Prevent timestamp jitter failing GPS data arrival check (#462)

This fixes a error condition that occurs if _time_last_gps is greater than _time_last_imu.
By checking time stamps at the fusion time horizon, we guarantee that this cannot happen because all observations must have a time stamp smaller or equal to _imu_sample_delayed.time_us before they are retrieved from the buffers.

EKF/control.cpp

@@ -612,7 +612,7 @@ void Ekf::controlGpsFusion()
 			_hvelInnovGate = fmaxf(_params.vel_innov_gate, 1.0f);
 		}
 
-	} else if (_control_status.flags.gps && (_time_last_imu - _time_last_gps > (uint64_t)10e6)) {
+	} else if (_control_status.flags.gps && (_imu_sample_delayed.time_us - _gps_sample_delayed.time_us > (uint64_t)10e6)) {
 		_control_status.flags.gps = false;
 		ECL_WARN("EKF GPS data stopped");
 	}