Add support for the Shearwater Swift GPS

[mikeller]

With the new Swift GPS transmitter, the GPS location of the dive entry
and exit points are stored in the opening and closing record number 9.

At the moment only the entry location is reported because the api only
supports a single location.

Cherry-picked-by: Michael Keller github@ike.ch
Signed-off-by: Michael Keller github@ike.ch

[torvalds]

Note that the the whole "single location" thing is somethign that the Garmin works around by just reporting multiple locations as free-hand text:

    // These seem to be the "real" GPS dive coordinates
    add_gps_string(garmin, "GPS1", &garmin->gps.SESSION.entry);
    add_gps_string(garmin, "GPS2", &garmin->gps.SESSION.exit);
    
    add_gps_string(garmin, "Session NE corner GPS", &garmin->gps.SESSION.NE);
    add_gps_string(garmin, "Session SW corner GPS", &garmin->gps.SESSION.SW);
            
    add_gps_string(garmin, "Lap entry GPS", &garmin->gps.LAP.entry);
    add_gps_string(garmin, "Lap exit GPS", &garmin->gps.LAP.exit);
    add_gps_string(garmin, "Lap some GPS", &garmin->gps.LAP.some);
    add_gps_string(garmin, "Lap other GPS", &garmin->gps.LAP.other);

    add_gps_string(garmin, "Record GPS", &garmin->gps.RECORD);

so that the information is at least there in the extra info, and people can see it (even if it doesn't then show up on the map)

The right thing to do would be to add a GPS sample, which woudl also solve the issue of dives where you peek up in the middle of the dive to see where the heck the boat is (or where the shore is when you're really lost ;)

[jefdriesen]

The right thing to do would be to add a GPS sample, which woudl also solve the issue of dives where you peek up in the middle of the dive to see where the heck the boat is (or where the shore is when you're really lost ;)

That's indeed one of the options I'm looking into. I have most of the implementation already prepared.

@mikeller Maybe wait a bit before merging this PR? There is at least one bug in the Shearwater GPS code reported that I'm still looking into. Apparently in some case the values can also contain 0xFFFFFFFF instead of zero. I suspect there will be some flag to indicate whether GPS data is available.