igc/writer: Added write_glider_type() method
igc/writer: Added write_glider_id() method
igc/writer: Added write_gps_datum() method
igc/writer: Added parameter defaults to write_gps_datum() method
igc/writer: Added write_firmware_version() method
igc/writer: Added write_hardware_version() method
igc/writer: Added write_logger_type() method