Produces calls of gene essentiality using an adaptation of the method described in Langridge et al.
Genome Research 2009 and Barquist et al. NAR 2013. A loess curve is fit to the distribution of insertion
indices, and used to identify the minima between the 'essential' and 'non-essential' distributions. These
distributions are then used to fit gamma distributions, which are then used to calculate log-odds ratios,
which are used to determine an insertion-index threshold for gene essentiality. Note that this analysis
requires a saturated mutant library, and is not suitable for the analysis of data sets with low insertion
density. The script produces a number of diagnostic plots which can be used to verify that this condition
has been met.