Expand section on filter

[jayware9]

https://github.com/DataScienceScotland/intro_to_r/blob/8a8e5bd0dbf7db188065fcceb5e45110567f5ffa/session1/intro_to_r_training.Rmd#L771

& and == is used without explanation, and the example is combining two verbs (summarise and filter) which may be confusing

I'm not sure that we can assume knowledge of logical operators (particularly as the syntax isn't the same as SAS), so we might be worth expanding on this of showing filter using AND, OR, and NOT. Also spending a bit more time here hopefully would reinforce the idea that tidyverse is about taking a dataset, doing something to it and getting a new dataset out.

Might be worth adding in some exercises for using filter as well.