Clean up early sessions

[briatte]

• [ ] dsr-03/01-covid-income
  • [ ] better document the code
  • [ ] explain merging in detail (use code and comments from dsr-04/01-debt)
  • [ ] hard-code OECD membership (document source in README)
  • [ ] add ggsave at end
• [ ] dsr-03/02-eu-mood
  • [ ] lose countrycode dependency?
• [ ] dsr-04/01-debt
  • [ ] remove merge
  • [ ] adjust README accordingly
  • [ ] focus on building line plots, ending with small multiples
• [ ] dsr-04/03-anscombe
  • [ ] cover base plots
  • [ ] cover lattice?

[briatte]

Replaced by #9, #10, #12 and #25