Edge curve is not preserved by saving, applying rewrites, etc.

[RazinShaikh]

The curve distance for the edge is stored in EItem but not in side the PyZX graph. As a result, the curve information is often lost such as when applying a rewrite or saving to a file. One possible fix would be to introduce edge_data dictionary in the PyZX graph class. This is similar to vdata for vertices, which is currently supported.