Fix error in epirange list check

[brookslogan]

• Fix this line to call is.list on the appropriate argument and change the non-working nested sapplys to something that makes sense (probably a single purrr::map_lgl). I encountered this when trying to feed a single Date object in.
• Fix any downstream behavior, e.g., in preparing the query, which may not actually accommodate, e.g., a list of epiranges.
• Add tests for this case.
• Update docs.