Minor adjustments to load libary directly

[manpen]

In the still pending PR for NetworKit I decided to copy the code to modify it slightly. However, actually not many changes are necessary:

1.) I need to be able to pass a PRNG reference rather than a seed
2.) I need to be able to stop the generation prematurely (NetworKit stops algorithms on SIGINT)

The first one is easy: I could add a SpatialTree::generateEdges(std::mt19937_64&); overload.
Additionally I would like to provide std::vector<std::mt19946_64*> overloads to generateWeights and generatePositions.

For the second I would add a template <typename StopCallback> to the SpatialTree which would default top a "never-stop" predicate and hence should be removed by the compiler if it remains unused.

Would you accept such a PR? Then we could use the library directly as a git submodule.

[chistopher]

1.) Fine with me
2.) A bit ugly, but I have no better solution.