Revamp the genome coverage to be stored in the chromosome object not the
module.
Extend the known length of a chromosome if we're given a read which is
beyond the end of the known current length.
Fix bug in line graph when minY is not 0
Change genome coverage to operate on Z-scores per chromosome so that
different chromosome densities don't mess up the plot.
Remove debugging from chromosome density module.
Improved the calculation of the genome density.
First attempt at a separated line graph for the genome coverage plot.
Improved the separate line graph to show a centre line and not show the
legend box since it's not needed any more.