Add user data to each point

This adds a user data field to the `OsmGpsMapPoint` type. This user data field can be used to store a pointer (or some arbitrary number) and can be used to link this point to other data structures which holds information about this point.

This adds four new functions to the `OsmGpsMapPoint` type (I hope the names are self-explanatory):
osm_gps_map_point_new_degrees_with_user_data
osm_gps_map_point_new_radians_with_user_data
osm_gps_map_point_get_user_data
osm_gps_map_point_set_user_data

Please let me know what you think of this addition.