Updating `MsBayesConfig` class.

Updating `taxa` attribute to be a list rather than a set to maintain the
order of the taxa as they are in the config file.
Updating `plotting` module.

Adding code for generating plots of marginal divergence times from the
results in the posterior summary file of a model that samples over
ordered models of divergence.

Adding `ErrorData` class to store the data and perform the plotting.
Adding the `error_data_list` attribute to `ScatterPlot`, along with the
`_plot_error_data` method, so that the `ScatterPlot` class can be used
to generate the divergence-time plot.

Also, adding the `get_marginal_divergence_time_plot` function that wraps
up the creation of the plot via `ScatterPlot`, and sticks the resulting
`ScatterPlot` instance into an instance of `PlotGrid`, and returns the
`PlotGrid`.
Removing debug print statements from `plotting`.
More hacking of `plotting` module.

Adding `x_label_size` and `y_label_size` attributes to `ScatterPlot`
class for controling the font size of the x and y labels.

Updating `get_marginal_divergence_time_plot` function to allow control
of x and y label font sizes. Also, creating a `Ticks` instance for the
x-axis. This keeps the x-axis from switching to a serif font when
matplotlib is setup to `usetex`.