[Enhancement] Implement the gspan algorithm for frequent subgraph mining

[Tixierae]

That would be nice. The reference is: Yan, X., & Han, J. (2002). gspan: Graph-based substructure pattern mining. In Data Mining, 2002. ICDM 2003. Proceedings. 2002 IEEE International Conference on (pp. 721-724). IEEE..

It is already implemented in matlab: http://www.nowozin.net/sebastian/gboost/

There used to be an R package implementing gspan: http://www.csc.ncsu.edu/faculty/samatova/practical-graph-mining-with-R/PracticalGraphMiningWithR.html but it is not maintained anymore, it was removed from CRAN, and it relies on igraph0. Also I'm not sure whether it is 100% reliable.