Complete docstring for condensation_graph()
Unify inclusion of tr1 headers based on libstdc++ version

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.
Remove unused variable from blockmodel.hh
Implement Graph.purge_vertices() via graph copying in O(V + E) time
Fix docstring in minimize_blockmodel_dl()
Fixes bug when edge/vertex_hist are called for non-scalar types