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Adds the gapminder data (closes #5), in both shapes so either teaching style works.
Tidy (long) — data/gapminder.csv, 1704 rows: country, year, pop, continent, lifeExp, gdpPercap. One row per country-year; suits plotnine and groupby exercises.
Wide — data/gapminder_all.csv and data/gapminder_gdp_{africa,americas,asia,europe,oceania}.csv. Matches Software Carpentry python-novice-gapminder, so its episodes can be taught unchanged.
Docs — data/README.md records columns, row counts and provenance for the gapminder files and the existing Portal surveys data. Top-level README gains a ## Data section.
Data copied verbatim from the Carpentries lesson repos at pinned commits (swcarpentry/python-novice-gapminder @ 5d886a8, swcarpentry/r-novice-gapminder @ 61dff93). Gapminder data under CC-BY 4.0. ~76 KB added.
Verified: all seven CSVs load with pandas — no nulls, 142 countries, years 1952–2007.
Consumed by training-environment-jupyter-python-app v0.5.0.