Fix remaining issues with pickling in Python 3
Fix computation of normalized Laplacian matrix
Switch from boost::adjacency_list<> to custom made and lightweight adj_list<>

Dumping boost::adjacency_list<> improves memory usage by a factor of
two, and also slightly improves performance in some cases.
Fix BlockState.min_dist()
Fix output of docstring tests.
Enable -Wextra by default during compilation.