EKF: Improve use of position and velocity observation noise parameters

Fix bug where noise parameters were not being squared when calculating the observation variance.
Use GPS reported speed accuracy to set velocity observation noise and use the parameter to set the minimum.

EKF/estimator_interface.cpp

@@ -137,6 +137,7 @@ void EstimatorInterface::setGpsData(uint64_t time_usec, struct gps_message *gps)
 		memcpy(gps_sample_new.vel._data[0], gps->vel_ned, sizeof(gps_sample_new.vel._data));
 
 		_gps_speed_valid = gps->vel_ned_valid;
+                _gps_speed_accuracy = gps->sacc;
 
 		float lpos_x = 0.0f;
 		float lpos_y = 0.0f;