Add proper covariances to NavSatFix

- Calculated GPS covariances based on http://answers.ros.org/question/10310/calculate-navsatfix-covariance/?answer=15197#post-id-15197
- Use lat_fused and long_fused for less drifty values
what does hdop stand for, @garyservin? I'm curious. Can you explain the calculation in a comment, for easier maintenance moving forward?