[BioPython] Spatial clustering

[Shu-Hsien Sheu]

Sorry but I forgot to mentioned this. The clustering code is using an 
algorithym that creats an appropriate number of clusters such that the
maximum distance between the hub of a cluster and any of its 
memebers(the cluster radius) is smaller then half of the average 
distance among all of the existing hubs. Also, an upper limit of the 
cluster radiusis was introduced to account for the physical dimentions 
of the molecule.