[JOSS Review] Functionality documentation: linting/style

[davidegraff]

cc openjournals/joss-reviews#4337

The code is documented and largely formatted with flake8 but there are a few nitpicks:

• $ flake8 --ignore E501 RIAssigner > flake8.log still outputs some minor errors:
  (flake8.log)
• asserts are used when a ValueError should be used instead (e.g., 1, 2, and 3. asserts should be avoided for client-side error-checking (i.e., invalid input values) because they can be entirely disabled

[hechth]

@davidegraff thank you very much for the useful hint - I wasn't aware of that regarding asserts - will fix that accordingly!