Updating `ModelProbabilityEstimator` worker class.

Adding `shared_div_summary` dict as an attribute. The class now keeps
track of the estimated probability that any number (from 2 to ntaxa) of
random taxa share a divergence event.
Updating `ModelProbabilityEstimatorTeam` class.

Adding `shared_div_summaries` and `shared_div_probs` dicts as
attributes. These are used to keep track of the estimated probability
that any number (from 2 to ntaxa) of random taxa share the same
divergence event.
Updating classes of `parsing` module.

Updating `OrderedDivergenceModelSummary`,
`OrderedDivergenceModelResults`, and `OrderedDivergenceModelCollection`
classes of `parsing` module. Adding `npairs` attribute to all three.
Also adding vetting of `taxon_indices` arguments passed to
`OrderedDivergenceModelCollection.prob_of_shared_divergence() method.
Adding `arg_is_positive_int` to `argparse_utils`.
Adding scripts.

Adding `dmc_plot_results.py` and `dmc_prob_shared_divergence.py` scripts
to the package's `scripts` directory.