This moves the entire algorithm into C++, and includes a faster
initialization heuristic.

This avoids a collapse with std::scoped_lock.

This fixes issue #511.

This modifies the MCMC code to allow simultaneous update of
all hierarchy levels, improving equilibration speed.

This fixes compilation with older GCC versions (ca. 5.0.0).

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

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