renamecols function for transform

[jtrakk]

I would like to pass a renamecols function to transform. innerjoin takes a renamecols function and transform takes a renamecols::Bool. I want to replace the Bool with a function parameter that defaults to column_function(c) = "$(c)_function" and can be replaced with identity for users who currently pass renamecols=false.

[bkamins]

This is a good proposal in line with some other proposals to allow dst to be a function in src => fun => dst, see #2876.
We will just need to work out the best approach.

[jtrakk]

Your proposal is better than mine. Closing.