This also adds the 'ignore_degrees' parameter.

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.

To specify fully random moves, one should set the parameter "c" to
"float(inf)", as explained in the docstring.

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.