warning before returning dups terminates function
- If duplicates were found in check_peaklist, the return would terminate the function before the error message of the stop() would be reached and the user would have no information where the error occurred. An alternative could be rlang::abort()
- Added warning when converting from data.frame to data.table
- Changed class(pl) to is.data.table(pl) for cleaner query, similar for is.data.frame; Not sure about speed.