Corrected covariance issue, topic names, velocity convention and misc other.

- All topics are now published on a private node handle.
- Velocities are published as stamped twist messages with covariance. Angular components are unsused.
- `hAcc`, `vAcc` and `sAcc` are used to generate diagonal covariances.
- Velocities use the correct convention: X-Y-Z = East-North-Up.
- 2D **or** 3D fix correspond to `STATUS_FIX` (previously only 3D).
- `fix` and `fix_velocity` are time-stamped synchronously, using the `iTOW` to check arrival times.
- Added options for changing the CFG-NAV5 settings (see above).
- Added support for `diagnostic_updater`.
- _"received ACK"_ messages are elevated to debug level 2.
- Corrected issue where baudrate was not set correctly using rosparam.
- Corrected issue where socket destructors were not called.