rewrite case study to bring tidy data to third day of course

[stijnvanhoey]

Most issues from jorisvandenbossche/DS-python-data-analysis#144 are tackled, but the following is something we have to decide on and tackle:

General idea: create case study without reshaping (eg for seaborn you don't need reshape if your data is already tidy), and leave reshape/tidy for third day, while on second day we can already practice groupby and seaborn