Add 'challenge' and 'solution' tags to cells

[pansapiens]

The instructor vs. student notes use these tags to keep solutions hidden in the student notes.

This must be done in Jupyter Notebooks since there is no UI (yet) in Juypter Lab.

[pansapiens]

• [x] intro
• [x] working with data
• [x] indexing
• [x] loops
• [x] plotting with plotnine (ggplot)