Add support for k-cores
Add support for the computation of k-cores, computed by pruning all the vertices (with their respective edges) with degree less than k. To each vertex it should be assigned a number, representing the highest k-core it belongs to.
See: http://en.wikipedia.org/wiki/K-core, http://arxiv.org/abs/cs.DS/0202039 (contains algorithm), [http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PLEEE8000073000005056101000001&idtype=cvips&gifs=yesk-core k-core (bootstrap) percolation on complex networks: Critical phenomena and nonlocal effects]