Perform reading-level analyses of the datasets; store in metadata

[dogweather]

See:

• https://www.geeksforgeeks.org/readability-index-pythonnlp/
• https://github.com/wimmuskee/readability-score
• https://github.com/cdimascio/py-readability-metrics
• https://pythonawesome.com/score-the-readability-of-text-using-popular-readability-metrics/