[WIP] Regulal spatial clustering

Algorithm from Senne et al [1]. Roughly the same complexity as k-centers, but probably superior. The code needs some work to be more efficient, because building up the `cluster_centers_` list element by element and then calling `np.array()` on it for every distance calculation is a lot of unnecessary copies. Also, this won't work for md.Trajectories yet.


[1] http://pubs.acs.org/doi/abs/10.1021/ct300274u