Creates a data frame that is useful for filling a spreadsheet, when you just
need a sheet to experiment with. The data frame has n rows and m columns
with these properties:
Column names match what Sheets displays: "A", "B", "C", and so on.
Inner cell values reflect the coordinates where each value will land in
the sheet, in A1-notation. So the first row is "B2", "C2", and so on.
Note that this n-row data frame will occupy n + 1 rows in the sheet,
because the column names occupy the first row.