Add option to use GBIF downloads in addition to search

[sckott]

waiting to hear back on https://discourse.gbif.org/t/limit-number-of-occurrences-returned-in-occurrence-downloads/2368

[sckott]

work on branch gbif-download

[sckott]

Big problems to solve:

• how to translate rgbif::occ_search/occ_data syntax to rgbif::occ_download syntax and vice versa. Once that is done, we can probably then accept either syntax here and translate as needed. there's queries you can do with download you simply can not do with search, e.g.,elevation > 100 - Thus, for the happy path you'd want to to find the set of queries that can be done for both download and search. Or, if we just use the download interface, then for "explore" queries (still don't know how we'd define that) use search endpt, and only allow queries that can be done with search.
• how to update output of occ() with gbif download results? perhaps a method on x$gbif if x is an occ object, so perhaps x$gbif$update(). If the download is ready yet, then nothing is changed. If download is ready, download data, and pull data into data.frame.
• if goal is to use search endpoint to "explore", then download endpoint to get final data, how does that get translated to the R user interface? not clear right now