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

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

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.