Functionality to retrieve (accepted) taxa of the GBIF backbone starting from a checklist

[damianooldoni]

The functionality to get (accepted) taxon from the GBIF backbone starting from a checklist has been proposed to GBIF. See code in ropensci/rgbif#746 (comment). Agreed to set this feature request on hold. However, based on request of @soriadelva, I think we should add this function to the trias package so that it is findable. When rgbif will implement it, we can deprecate our trias function and refer to the correspondent rgbif function.

To do:

• Write function
• Write documentation
• Write unit-tests