Updating `ListConditionEvaluator` class.

Adding `index_label` option for `__init__` method. Also, adding
`pretty_expression` attribute, which is the `expression` string with the
`index_label`s substituted in for the list indices.
Adding `dmc_posterior_prob.py` script.
Renaming script.

Renaming `dmc_posterior_prob.py` script to `dmc_posterior_probs.py`.
Updating `setup.py`.

Making an explicit list of scripts to install/link. Excluding
`dmc_prob_shared_divergence.py` script from this list, as it has been
superseded by `dmc_posterior_probs.py`.