HDOP inclusion tweaks for GPS and working TTNMapper examples

[Dukat-Gul]

Included are a reference python CayenneLLP and TTNMapper compliant GPS tracker/transmitter.

CayenneLLP just sends Latitude, Longitude and Altitude - which is acceptable to TTNMapper however they ideally like to have a HDOP (Horizontal Dilution of Precision) reference for positional accuracy. To achieve this the GPShandler.py has been modified to include SKY objects as well as the TPV (Time Position Velocity) Object.

[BNNorman]

GPShandler now caches all GPS sentences so you can get SKY and TPV data and use it how you wish rather than adding more instance variables to GPShandler. e.g.

# create a Dragino object and join to TTN
D = Dragino("dragino.toml", logging_level=logLevel)
D.join()

def checkGPS():
    global gps,D

    tpv=D.get_msg_class("TPV")

    if tpv is not None:
        gps["lat"]=round(tpv['lat'],5)
        gps["lon"]=round(tpv['lon'],6)
        print("got gps TPV sentence")
    else:
        print("waiting for TPV")
        return False

    sky=D.get_msg_class("SKY")
    if sky is not None:
        gps["hdop"]=round(sky["hdop"],2)
        gps["sats"]=len(sky['satellites'])
        print("got gps SKY sentence")
    else:
        print("waiting for SKY")
        return False

    return True # TPV and SKY ok