This also re-organizes entropy() w.r.t. entropy_args.

This fixes issue #426

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

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

This includes support for sampling from posteriors of layered models
that are also overlapping and/or with edge covariates.

This fixes issue #325

This also fixes a potential problem with layered SBMs, where the move
probabilities would be incorrectly computed.

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.

This reverts commit df948de9.

As it turns out, some slightly older compilers (e.g. GCC 5.6) do not
like this.

This fixes issue #351