gps: fixed filter initialization bug around gps

- do not gps reference altitude to zero in case gps checks pass before the
filter initialized
- reset the filtered gps position and position derivative filters in case
we are in air or there is movement on the ground

Signed-off-by: RomanBapst <bapstroman@gmail.com>

EKF/ekf_helper.cpp

@@ -1866,3 +1866,9 @@ void Ekf::runYawEKFGSF()
 		yawEstimator.setVelocity(_gps_sample_delayed.vel.xy(), _gps_sample_delayed.vacc);
 	}
 }
+
+void Ekf::resetGpsDriftCheckFilters()
+{
+	_gps_velNE_filt.setZero();
+	_gps_pos_deriv_filt.setZero();
+}