Returns the Simpson index $D$: the
probability that two randomly sampled individuals belong to
different species. This is
$$1-\sum_{i=1}^S\left(p_i\right)^2$$
where $p_i$ is the relative frequency of the $i^{\rm th}$
species
References
S. P. Hubbell 2001.The Unified Neutral Theory of Biodiversity.
Princeton University Press.
F. He and X.-S. Hu 2005.Hubbell's Fundamental Biodiversity
Parameter and the Simpson Diversity Index. Ecology Letters, volume 8,
pp386-390. doi:10.1111/j.1461-0248.2005.00729.x
E. H. Simpson 1949.Measurement of diversity.
Nature, volume 163, p688