Replace data with df in the pandas chapter

[Scottan]

Replace the use of "data" with "df" to denote a DataFrame in the pandas section of the course, e.g. replace:

https://github.com/UoMResearchIT/programming_with_python/blob/main/episodes/07-pandas_essential.md#:~:text=data%20%3D%20pd.read_csv(%27data/gapminder_gdp_europe.csv%27%2Cindex_col%3D%27country%27)

with:

> df = pd.read_csv('data/gapminder_gdp_europe.csv',index_col='country')

"df" is the convention for a generic DataFrame, whereas the format of "data" is ambiguous.