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

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

This adds support for exact likelihood calculation, correct posterior
sampling of models with nonempty groups, much improved "distributed"
degree-corrected prior, as well as continuous edge covariates.

This is a significant refactoring of the statistical inference code,
with many cleanups and improvements, and is aimed towards future
extensibility.

The old community module is available as 'community_old' and will be
completely removed in the near future.