[entitynet] restructure entity networks
This completely restructures the entity network class, which now allows for iteration over the fragments of the different constituents (subdomains, entities, intersections, junctions). Moreover, one can extract connectivity information on intersections and junctions. The builder and writer classes are adapted accordingly, and are still used in the same way as before. The entity network class itself was modified in a non-backwards-compatible way, as the current implementation did not provide much functionality apart from passing it onto a writer, which can still be done in the same way.
Showing with 734 additions and 440 deletions