Add histogram plot of per sample heterozygosity

[kottewell]

An idea for the next release:
Calculate the proportion of heterozygous sites for each scat genotype and plot as a histogram to visualise outliers as a way to identify potentially contaminated samples. Samples with excessively high heterozygosity (relative to other samples) are likely to have been cross-contaminated with DNA from other individuals, either in the field or during lab procedures and may need to be removed from analyses.