Updating `plotting` module.

Overhauling `ErrorData` class and `get_marginal_divergence_time_plot`
function, so that horizontally and vertically oriented plots can be
handled. This also required a change to the `_plot_error_data` method of
`ScatterPlot`.

Beginning work on `UnorderedDivergenceModelPlotGrid` class, which will
(hopefully) plot the most common divergence models from a posterior.