Dealing with new alleles during append #31

It's unlikely we'll be able to predict all alleles ahead of time. The simplest approach is to append the new allele(s) to the variant_alleles array and to recode the genotypes in the input data. Note that this means we'll also need to detect inconsistent allele encodings and deal with those.

This seems like a reasonable line to draw for altering the data on append - so, we hard-require that the data has the right dimensions (#30), but we update the encoding as necessary.