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 fixes some strange bugs where deterministic position updates lead
to strange artifacts.

The object may be globally reset via the seed_rng() function.

This implements a graceful fallback in case GTK cannot be loaded, which
can (currently) happen simply by not having a environment DISPLAY
variable.

Images are now converted to png a posteriori for html output, but are
used as pdf for latex.

This allows pinning of individual vertices via the `pin` paramenter, as
well as string-valued color property maps, and some other minor
improvements.