add rank argument to atlas_species()

[shandiya]

It would be helpful to have the option of specifying a desired taxonomic rank when searching for and downloading occurrence records. This would be especially useful in cases where certain groups of organisms e.g. protists are very rarely classified down to species level, and more often at the family level or higher.

[mjwestgate]

In theory this is possible, as atlas_species() is built using the occurrences/facets/download, meaning we could swap the facets argument from speciesID to...something else. There are a few questions that I have about this:

• Does lookup=TRUE still work in this instance? This is necessary to 'fill in' further taxonomic information for each row. I suspect it would, but requires checking.
• What should the function be called? I'd suggest atlas_taxa() for consistency with search_taxa(), and have atlas_species() retained as a special case of atlas_taxa() where rank = "species".
• How would this work with new piped syntax? atlas_taxonomy accepts a rank argument within filter(), so it might be useful to port that over.

A final point; @acbuyan is working towards supporting a count argument in atlas_species(); while we're looking at syntax it would be worth tackling that question as well.