Use this to calculate the sum of branch lengths that are represented between two
communities. This measure is also known as the unique fraction. It can be used to measure
concepts of phylogenetic turnover. Parallelizable.
References
Lozupone, C., & Knight, R. (2005). UniFrac: a new phylogenetic method for comparing
microbial communities. Applied and Environmental Microbiology, 71(12), 8228-35.