Updating `probability` module.

Adding `Multinomial` class. Adding associated tests to
`test_probability.py`.

Also, moving `almost_equal` from `stats` module to `probability` module.
Updating `stats` module.

Removing `almost_equal` function (now in `probability` module).

Adding methods to `IntegerPartition` and `Partition` classes for
calculating prior probabilities.

Also, adding `get_integer_partition` method to `Partition` to create and
return an `IntegerPartition` instance from an existing `Partition`
instance.