Use old download keys!

[hannahlowens]

From @damianooldoni: "Let's say I had to trigger a download via rgbif instead of using your getGBIFpoints() function as I need to download all data of all species in a region instead of specifying a taxon/phylogeny. Well, it would be nice to allow me to still use the rest of the package functionalities, isn't? This can be another way to solve the dilemma between user friendly code and flexibility. How to do it? you can allow me to get my GBIF download by passing a GBIF download key or the GBIF DOI to getGBIFpoints() and so to prevGBIFdownload()!"