Ensure checksum is 2 digits (#143)

Fix bug where some L76 messages are ignored due to the checksum being one digit long (i.e. 'F') vs. the expected 2 digit length ('0F')

shields/lib/L76GNSS.py

@@ -113,7 +113,7 @@ def _checksum(self, nmeadata):
         calc_cksum = 0
         for s in nmeadata:
             calc_cksum ^= ord(s)
-        return('{:X}'.format(calc_cksum))
+        return('{:02X}'.format(calc_cksum))
 
     def write(self, data):
         self.i2c.writeto(GPS_I2CADDR, '${}*{}\r\n'.format(data, self._checksum(data)) )