This fixes issue #624

This also adds the missing Graph.get_total_degrees() and missing
documentation.

This fixes issue #596

This improves performance of the Python API.

It now accepts edge descriptors from all graph views and invalidates
them after removal.

This essentially replaces "neighbours" by "neighbors" throughout.

Aliases using the British spelling (e.g. Vertex.out_neighbours()) are
preserved for compatibility.

This changes the underlying adjacency matrix implementation to a single
mixed in + out edge list for each node, instead of separate ones. This
simplifies the undirected_adaptor<> so that its out-edge iterators
become trivial, and also faster.

Now algorithms rely more on parallel_{vertex/edge}_loop() and global
the paralellization threshold is honored.

This introduces support for returning a generator in
subgraph_isomorphism(), so that all matches do not need to be kept on
memory simultaneously.

This fixes issue #255.

This fixes #189

This fixes a bug where equal edge descriptors would be considered
unequal. Additionally, now Vertex and Edges can be ordered (via >, <,
>=, <=).