Updating Partition class.

Updating `Partition` class of `stats`. There were errors for initiating
a new instance as a copy of of an existing instance.  This commit should
fix this.
Adding test for `Partition` class.

Adding test for creating a new instance from existing instance.
Adding/updating classes in `stats` module.

Adding `element_vector_iter` method to `Partition` class. Adding
associated test to `test_stats.py`.

Adding `ListConditionEvaluator` class to `stats` module. Adding
associated tests to `test_stats.py`.