Implement graph-tool

[mannbach]

We should provide an alternative to NetworkX to improve performance.

Some requirements from the top of my head:

1. Using graph-tool should be optional. Only when people specifically select it, we should require it to be installed.
2. It should be hidden behind an abstract network layer (which needs to be implemented for this purpose). This abstract layer describes the network and the functions we need (e.g., for adding nodes, labels and analyses). From the abstract network layer, inheriting classes can implement the access to NetworkX and graph-tool objects respectively.