Improvements to lpe for flow and gps. (#5401)

.../local_position_estimator/BlockLocalPositionEstimator.cpp

.../local_position_estimator/BlockLocalPositionEstimator.hpp

src/modules/local_position_estimator/params.c

src/modules/local_position_estimator/sensors/flow.cpp

src/modules/local_position_estimator/sensors/gps.cpp

src/modules/mavlink/mavlink_messages.cpp