Substitute values in summary rows

[RemiDumas]

Prework

• Read and abide by gt's code of conduct and contributing guidelines.
• Search for duplicates among the existing issues (both open and closed).

Proposal

sub_values() function only allows to substitute values in body rows. It'll be useful to hide an entire column keeping the column label and the empty cells.

[olivroy]

I also agree this would be useful. Please provide more details of your use case!

As a workaround, you can define a function that substitute your desired values (See example in #1440)