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).
more to come...