_parse_gpgsa and _parse_gpgsv are unused. #51
Description
The methods _parse_gpgsa() and _parse_gpgsv() are never called from the message parser update(). The also leads to the following variables never being updated sel_mode, fix_quality_3d, sat_prns, pdop, hdop, vdop & satellites_prev.
The GSA (GPS DOP and active satellites) and GSV (Satellites in view) messages looks like they might provide useful extra information if received.