Improve Module 2 hands-on solutions

[jack89roberts]

I think the instructions could also be improved/clarified, but the solutions would benefit from some more prose and I think some of the solutions use unnecessarily complex code (especially if we have Python beginners on this course), e.g.

missing = df.groupby("eqls_wave").apply(lambda g: g.isna().mean() * 100).round(3).transpose()

could be:

missing = df.isna().groupby(df["eqls_wave"]).mean()