Updating `get_marginal_divergence_time_plot`.

Making `usetex` setting of `matplotlib` a boolean option for the
function, so that it can be turned off if strings have syntax that
conficts tith TeX.
Updating `dmc_plot_results.py` script.

Adding `usetex = False` option to `get_marginal_divergence_time_plot`,
and fixing `margina_times_plot` -> `marginal_times_plot` typo.