Enable GSV (satellites in view) parsing.
Rewrote the GSA and GSV parsing to handle each satellite system
(talker) separately.
- self.sats now uses keys based upon the talker and satellite
number, eg. GL67 for GLONASS #67, GP7 for GPS #7
- When the end message of a GSV sequence is received, eg. 3 of 3,
all previous records in self.sats matching that talker are removed
before adding the updated ones.
- self.sat_prns stores the last satellite IDs that were used for a
fix and returned in the most recent GSA sentence. They will be
from only one Satellite system and should have a record in
self.sats .
