Mode_indicator of GNS instead of the gps_qual from GGA sentence

[LukasBaecker]

The GNSS module sends information formatted along the NMEA-0183 standard. We are currently using the Lat/Lon position from the GNS message which contains GNSS data. For data quality checks we are further using the gps_qual of the GGA massage which only refers to GPS. The mode_indicator of the GNS message contains the mode of every single satellite system.

Todo:

• use the mode_indicator instead of the gps_qual
• check, if it is enough if a single satellite system gives RTK fixed data (e.g. mode: 'RNNN) or if all need to be fixes (mode: 'RRRR') to get RTX-fixed LatLon positions
• test on a robot

[angelom93]

Looks very nice!