This fixes issue #322

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.

Make sure to preserve "ignore_degrees" even when the values are
non-Boolean (i.e. 2).

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.