Coupled-biased-random-walks

* [Added] version is exposed as `coupled_biased_random_walks.__version__`

* [Added] type hints
* [Changed] removed support for Python 2 and <3.7
* [Changed] updated dependencies to latest versions

* [Fixed] enforce feature weight and stationary probability sum normalization

* [Added] `CBRW.value_scores()` function to return individual value scores of an observation
* [Added] exceptions inherit from base `CBRWError` exception
* [Added] test cases for NaN values
* [Changed] README describes value_scores functionality
* [Changed] example.py is reorganized to include value_scores
* [Fixed] tests compare sorted lists