Module ClusterUtils

returns an ordered list of all nodes below cluster

the ordering is done using the lengths of the child nodes

 **Arguments**

   - cluster: the cluster in question

 **Returns**

   - a list of the leaves below this cluster

find the point in a cluster which has the smallest summed 
  Euclidean distance to all others

**Arguments**

  - cluster: the cluster to work with

  - dists: the distance matrix to use for the points

**Returns**

  - the index of the centroid point

splits a cluster tree into a set of branches

**Arguments**

  - cluster: the root of the cluster tree

  - n: the number of clusters to include in the split

  - breadthFirst: toggles breadth first (vs depth first) cleavage
    of the cluster tree.

**Returns**

  - a list of sub clusters