Best practice for the use of shapes

GTFS allows to publish shapes as it is an abstract thing. Technically it is even possible to share shapes between trips that do not have the same stop sequence, but follow the same route-over-the-road or route-over-the-rail and use parts of a shape. For long distance busses shapes.txt might be enormously dense, copying many highways and city entrances, while the opposite might also be true and the shape would be more something that is intended for map matching.

1. shapes.txt is currently a list, there is no file that defines the properties of a shape_id. I think we must introduce a new file to facilitate this, to allow to define if a shape is specific or abstracted, and is shared between unrelated stop sequences.
2. The way how shapes are designed there is no formal requirement that at a stop it must be split (hence a shape point) exist, because the stop_times.txt defines an _offset_. I think it would be a very good best practice that such point does exist, hence a straight road with three stops would not just have two shape points (at the beginning and end) but also a point on which the stop can be projected