Hyperedges enabled and api according to wishlist

[fjug]

more to come...

[fjug]

Ok, this pull request delivers the following features:

• Hyperedges in TrackGraph, networkx graph not held in TrackGraph any longer
• Empty creation of TrackGraph possible.
• Functions for adding nodes and (hyper)edges.
• One can add networkx graphs, even to already existing TrackGraphs
  • in this case, no node and edge duplicates will be added
  • attribute dicts will be merged
  • existing attributes will be overridden with the newly given values Test run through (including some new tests for the API we agreed on, including all features mentioned above).

[funkey]

Thanks a lot!