hector_pose_estimation: fixed wrong indicies bug in GPS pose publisher
hector_pose_estimation: make private variables protected to allow access from specialized pose_estimation classes
hector_pose_estimation_core: initialize reference values to NaN instead of 0.0 and added measurement/auto_* parameters consitently

Added parameters:
- gps/auto_reference
- height/auto_elevation
- baro/auto_elevation

Already existed before:
- magnetic/auto_heading

All auto_* parameters are true by default.
hector_pose_estimation_core: fixed rate conversion to nav frame for the state message