Avoid duplicating content with base R/tidyverse

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This course has a reasonably long section on subsetting of data frames that mostly mirrors the subsetting of vectors. While it is certainly useful to know how to do that it is then mostly ignored in favour of dplyr's select() and filter() methods.

Since there is more content than we usually manage anyway, I would like to propose cutting most of the base R subsetting for data frames. It may be useful to retain a much smaller section that essentially says, look, you can subset data frames in almost the same way as vectors.

Teaching one way to do things should be enough for a course like this and sticking to the tidyverse way as much as possible should help to avoid confusion.

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A related issue is the data frame / tibble distinction. Since read_csv() is used to read the data learners never actually interact with data frames directly and much of the text in that section is somewhat inaccurate in that respect.

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And while we are talking about using the tidyverse where possible, the section on factors should probably be converted to use the forcats package.