Adding `get_partitions_from_posterior_sample_file`.

Adding `get_partitions_from_posterior_sample_file` function to `parsing`
module. This is now used by the `_parse_posterior_file` method of
`UnorderedDivergenceModelResults` and `OrderedDivergenceModelResults`
classes.
Updating classes in `stats` module.

Updating `ListConditionEvaluator` class. Adding checking of expression
to make sure it only has relation operators.

Adding `get_condition_count` method to `Partition` class. Counts the
number of paritions for which a `ListConditionEvaluator` instance
evaluates to true.

Adding `get_condition_count` and `get_condition_frequency` methods to
`PartitionCollection` class.

Adding associated tests to `test_stats.py`.
Adding `DivModelSimulator` class to `workers`.
Adding `DivModelSimulatorTeam` class to `teams`.