Use of 'sheet' and 'varColumn' in `copyScanSelection()`

The optional arguments 'sheet' and 'varColumn' in function `copyScanSelection()` aren't used consistently, but sometimes plainly ignored, e. g. in

- line 71 `readxl::read_excel(codebook, sheet = "Codebook", skip = startRow-1, col_types = "text")` or
- line 82 `length(setdiff(unique(codebook[rw,"Variable"]), ""))`

but perhaps also on other occasions.

Locate copyScanSelection() and inspect every use of sheet and varColumn, starting with the read_excel call around line 71 and the codebook access around line 82. Confirm the intended arguments from the function's callers, then verify that all relevant uses consistently honor them.

Written by the indexing model from the issue text.