FR: user supplied function for detecting differences

[eutwt]

Currently versus:::not_equal is used. It should be possible to allow user-supplied functions, but the results would have to be checked once applied to each column

• Output must be logical and contain no missing values
• Output must be of the same length as the columns

Because the output of user-defined functions may take longer to compute, it would be good to have a progress bar using the {progress} package.