Updating div model plotting classes.

Updating `UnorderedDivergenceModelPlotGrid` and
`OrderedDivergenceModelPlotGrid` plotting classes to allow a
`time_multiplier` for scaling time. Also, updating
`NumberOfDivergencesSummary` plotting class to allow the plotting of the
Bayes factors alone.
Updating `dmc_plot_results.py` script.

Adding option "-m/--mu" CLI option for specifying the mutation rate in
order to rescale time in generations. Also, adding the output of a plot
of the number of divergence event Bayes factors (without the posterior
and prior plots).