Hector GPS/IMU sensor alternative

[woensug-choi]

@woensug-choi My understanding is two issues.

1. Inaccuracy of reported lat/lon.

• We should put together a minimal example that does not depend on any of our code to demonstrate the potential issue. If there is an inaccuracy, please report it as an issue to the Hector repository: https://github.com/tu-darmstadt-ros-pkg/hector_gazebo/tree/kinetic-devel/hector_gazebo_plugins
• If there is a fundamental error with the plugin, we should try to fix it for everyone instead of maintaining our own fork of this well used plugin.

2. Added feature of not publishing GPS when underwater. There are (at least) two ways that this could be addressed without the need to maintain our own fork of the Hector code
   a) We could add a standalone ROS node that republishes the GPS fix information only when the GPS antenna is above the water
   b) We could add a feature to the Hector source code and then submit a PR so that we are contributing to that code base.

Originally posted by @bsb808 in #28 (comment)