Large requests raise an error when Sys.sleep is called

[jpeacock29]

When large requests are made, CitationChaser pauses between requests to avoid overloading the LENS API. However, when Sys.sleep(60 - t) is hit, t is not defined (besides it's definition in base R as a matrix transpose...). I fixed this locally by replacing 60 - t with 20 (a fixed 20 second sleep period).

I can make a pull request with this fix, but I couldn't understand what the intended functionality was exactly. I noticed a t_ref was defined and measured the duration of the last API call.