added mean gene length to stats, but it isn't calculated yet.
calculating mean gene length
mrna, gene, seq now have get_num_introns()
mean intron length added to stats manager
mean cds length added to stats manager
added proportion of genome covered by genes to stats manager, phew
added proportion of genome covered by CDS to stats manager
mrnas per gene added to stats
exons per mrna added to stats
introns per mrna added to stats