Computes the treelikeness
References
BR Holland, KT Huber, A Dress, V Moulton (2002) \(\delta\) Plots: a tool for analyzing phylogenetic distance data Russell D. Gray, David Bryant, Simon J. Greenhill (2010) On the shape and fabric of human history Molecular Biology and Evolution, 19(12) 2051–2059
Russell D. Gray, David Bryant, Simon J. Greenhill (2010) On the shape and fabric of human history Phil. Trans. R. Soc. B, 365 3923–3933; DOI: 10.1098/rstb.2010.0162