Implement time, altitude functions and GLL latitude function

[kknives]

As of now, the functions to return time and altitude are unimplemented and just return an error code when called:

R24-Test-NMEA0183-Parser/gps.c

Lines 77 to 82 in 0393acf

	gps_error_code_t gps_get_time(gps_t gps_instance, struct tm* time) {
	return GPS_UNIMPLEMENTED;
	}
	gps_error_code_t gps_get_altitude(gps_t gps_instance, float* msl_metres) {
	return GPS_UNIMPLEMENTED;
	}

Also, the gll_get_lat_lon implementation is just a stub:

R24-Test-NMEA0183-Parser/gps.c

Lines 157 to 159 in ce16573

	gps_error_code_t gll_get_lat_lon(int* degmin, int* minfrac) {
	return GPS_UNIMPLEMENTED;
	}

1. Implement gps_get_time and gps_get_altitude for both GLL and GGA sentences using gps_get_lat_lon's implementation as a reference.
2. Implement gll_get_lat_lon

Also write tests against the implementation and place then in the tests directory.