convert impact_coding_on_a_single_column to transformer

[Aylr]

• [ ] convert to transformer
• [ ] audit and increase tests if needed

[glenrs]

@Aylr, I can do this. I noticed that we are separating the transformers into different files. Since this is not included in the transformers.py file it will not be converted to the separate files. Do you want me to wait until that is merged?

Also I noticed that there is another function "impact_coding_on_many_columns". Do you want this converted to transformer?