use a more relatable starting point

From feedback received during pilot lesson at CC@Home2020: the lesson material could be improved by starting with a more authentic project/code, e.g. a relatively simple Numpy analysis. It was suggested that this would be more likely to reflect the experience of the target audience, and could make the value of tools such as `virtualenv`/a `requirements.txt` file more immediately apparent later in the lesson.

Review the existing lesson material and the pilot feedback described in this issue. Compare its current starting point with a small, authentic NumPy analysis, and assess how that would introduce virtualenv and requirements.txt; done means the lesson has an agreed, revised starting point rather than only a general suggestion.

Written by the indexing model from the issue text.