This reverts commit cf122227.

This replaces a custom implementation of Ullmann's code.

Now default constructed edge descriptors are impossible to occur in
regular graphs.

This switches the edge_descriptor type in adj_list<> from std::tuple to
a custom struct.

This also implements direct manipulation of OpenMP settings (schedule
and number of threads) via python.

This makes edge deletion O(1) if fast edge deletion is enabled for the
graph.

Ensure in/out_edge/adjacency_iterator and associated are random access in adj_ist<> and UndirectedAdaptor<>