split retrieve_IDs off from retrieve_data()

[katrinleinweber]

Extract to different function? If yes, by scraping IDs from paged URL returns (official examples), or by storing the URLs as intermediate result, plus providing helper functions to narrow that result down to the IDs?

Or, implement as an internal loop-back in retrieve_data(…, searchType = "taxon") based on new parameter taxon_data = TRUE?

• ask, whether only taxon search can return multiple IDs

[katrinleinweber]

While working on #16 I noticed that it's difficult to explain why something called …data returns an ID vector. I already implemented #21, so I might as well:

• align the function names entirely,
• move them with the aggregate… helpers into different files, and
• let one call the other, when needed.

[katrinleinweber]

Now part of #54.