Improve Deaton 2021 (dsr-03/01-covid-income)

[briatte]

dsr-03/01-covid-income needs more structure, and some steps need to be fully documented, e.g.

• [x] better document the code
  • [x] locale()
  • [x] as.Date()
  • [x] skip in read_excel
• [x] merging
  • [x] explain merging in detail (use code and comments from dsr-04/01-debt)

Also:

• [x] The header of the script comes from QUANTI2 and should be changed.
• [x] The "quick check" part should show str, head and tail, View and glimpse.
• [x] hard-code OECD membership: document source in README
  • [x] remove dependency to memberstates::memberstates$oecd$iso3c
• add ggsave at end