Updating partition classes.

Updating `Partition`, `IntegerPartition`, and
`IntegerPartitionCollection` classes. Overhauling dpp draw methods of
`Partition`, and adding similar methods for other divergence model
priors: `psi_multinomial_draw`, `psi_multinomial_draw_iter`, and
`uniform_integer_partition_draw_iter`.

Adding method `get_random_element_vector` to `IntegerPartition` which
returns a randomly ordered element vector consistent with the integer
partition. Also adding `get_all_integer_partitions` method to
`IntegerPartition`, which returns an `IntegerPartitionCollection` of all
possible integer partitions for a given number of elements.

Adding `draw` method to `IntegerPartitionCollection` that allows an
integer partition to be randomly drawn (with replacement) from the
collection.
Updating `test_stats.py`.

Updating tests of dpp draw methods of `Partition` class.
Adding `ModelProbabilityEstimator` class.

Adding worker class `ModelProbabilityEstimator`. This class samples from
the prior on divergence models as specified in an msBayes config file.
The main purpose of this class is to obtain Monte Carlo estimates of the
prior probability of one divergence event.
Adding `dmc_estimate_prior_probs.py` script.

This script allows fast, multi-processed Monte Carlo estimates of prior
probabilities of divergence models.