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

This fixes issue #325

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.

This implements a general wrapper for dense_hash_map and dense_hash_set,
that falls back to unordered_map and unordered_set, if sparsehash is not
enabled.

This closes #225