DBindex: DBindex
In chickn: 'Compressive' Hierarchical Kernel Clustering Toolbox

Davies Bouldin index computation.

1	DBindex(X_cumsum, rowInd, Clusters, C, cluster_size, DistInter)

`X_cumsum`	is a Filebacked Big Matrix, which contains column matrix cumulative sums.
`rowInd`	is a vector of the row indeces
`Clusters`	is a list of cluster assignment
`C`	is a matrix of the cluster centroids
`cluster_size`	is a vector of the cluster sizes
`DistInter`	is a Filebacked Big Matrix, which stores distances between a cluster centoid and signals in the cluster