Fast hierarchical clustering method - PHA

Performs fast hierarchical clustering using PHA method. The function will produce a hierarchical cluster tree (Z) from the input distance matrix. The output Z is similar to the output by the Matlab function 'linkage'.
[Key features]
1. Faster than matlab linkage function.
2. Excellent performance for clusters with mixture of Normal distributions.
3. Ideal tool for producing phylogenetic trees.

[Reference]
Yonggang Lu, Yi Wan. (2013). "PHA: A Fast Potential-based Hierarchical Agglomerative Clustering Method," Pattern Recognition, Vol. 46(5), pp. 1227-1239.