Implement NMEA GBS message

[SlashDevin]

Although the GST sentence reports the standard deviation of the lat/lon/alt errors, the GBS sentence provides additional insight into the GPS device's Receiver Autonomous Integrity Monitoring Algorithm (RAIM) process.

Currently, the GSV sentences identify which satellite IDs are not being used, have a weak signal, or are close to the horizon. There are also the qualitative DOP values that provide a qualitative measure of how the current constellation.

Parsing the GBS sentence would require additional fields in fix structure.

First requested in Issue #87.