Adding `prob_clustered` to `PartitionCollection`.

Adding `prob_clustered` method to `PartitionCollection` class.  Adding
associated test to `PartitionCollectionTestCase` of `test_stats.py`.
Adding classes to `parsing` module.

Adding `OrderedDivergenceModelSummary`, `OrderedDivergenceModelResults`,
and `OrderedDivergenceModelCollection` classes to `parsing module. These
classes enable parsing of ordered divergence model results.

Adding associated tests to `test_parsing.py`.
Adding `OrderedDivergenceModelPlotGrid` to `plotting`.