This script deletes monomorphic loci from a genlight {adegenet} object
A DArT dataset will not have monomorphic loci, but they can arise, along with
loci that are scored all NA, when populations or individuals are deleted.
Retaining monomorphic loci unnecessarily increases the size of the dataset
and will affect some calculations.
Note that for SNP data, NAs likely represent null alleles; in tag
presence/absence data, NAs represent missing values (presence/absence could
not be reliably scored)