Clarify whether show_rounds_country() is an AND or OR logical

[leeper]

The show_rounds_country() function takes a numeric vector of ESS rounds (as integers) and returns participating countries. It's not obvious from the documentation, however, whether the countries that are returned are those that participated in any of those rounds or the set of countries that participated in all of those rounds. Some clarity might be useful or, alternative, two functions (or a new argument to this function) to disambiguate might be appropriate.

[cimentadaj]

See https://github.com/cimentadaj/ess/commit/76007c5ac35161629a3eefed4a941a9a726dee0d where I improved documentation as to make it clear with some examples. Adding a OR type of function could be handy, and something that I will add for next releases yet I don't think it's urgent. Users can find a work around around along theses lines purrr::map(ess::show_rounds(), ess::show_rounds_country).