Added built in allele frequencies for agenotype simulations

This deals with the subject of issue #14, but instead of
allowing the user to directly specify a allele frequency
containing file, the frequencies are hard coded. The optimal
things would be to use the Ensemble perl API (if that is possible)
to retrieve allele frequencies dynamically. But two of the variants
are missing population data currently (allele frequencies for these
were taken from a couple of articles).