Use `readr::read_csv()` throughout the lesson

[Teebusch]

Currently, "starting with data" introduces base R's read.csv(), but "Manipulating data" then switches to readr::read_csv(). SInce R 4.0 the default behavior of both functions is to read strings as character vectors, so there is little reason to introduce both. For beginners the confusion that this creates is arguably larger than the insights gained from it, as evidenced by some of the issues here.

This is related to a number of issues and might close some of them:

609 #471 #608 #553 #555 #559 #616 #645

The main change is:

• move to "starting with data" section: explanation of packages and package loading, introduction of tibbles,
• remove discussion of "stringsAsFactors" (except for a short mention of former default behavior)
• adapt challenges and code examples
• smaller rewriting here and there.

[fmichonneau]

would another @datacarpentry/r-ecology-lesson-maintainers be willing to also review this PR? Thanks!

[mondorescue]

would another @datacarpentry/r-ecology-lesson-maintainers be willing to also review this PR? Thanks!

@fmichonneau I'll review this PR as a second.

[fmichonneau]

Thank you both!