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.

Xin Sun authored Dec 21, 2013 and Tiago Peixoto committed Dec 21, 2013

In case google sparsehash uses a non-standard prefix, e.g., in Homebrew.

Avoid hash of pair<vertex_t, vertex_t> types.

This greatly improves the performance of the previous algorithm, and
includes support for hierarchical models.

Dumping boost::adjacency_list<> improves memory usage by a factor of
two, and also slightly improves performance in some cases.

This enables the inclusion of the non-boost tr1 headers if a recent
enough version of libstdc++ is detected. This als fixes a bug with
clang when sparsehash pulls unwanted tr1 headers.

The condensation_graph() function now returns a property map which
contains the community values themselves for each vertex.

This enables support for older versions of GCC (>=4.2), by falling back
to the boost tr1 implementation when the GCC version is less than 4.4.

Edges were not properly indexed after graph generation.

Thins changes the graph filtering code slightly to wrap graph types with
GraphWrap, which automatically updates the edge index list when edges
are removed and added to the graph.

This also changes how graphs are passed to algorithms, which is now by
reference instead of pointer. (hence this touches lots of code, but
changes are trivial)