Addition of functionality to go back and undo previous classification

[DDelbarre]

When running domain_mapper a user could very easily enter the wrong code, and even though they might immediately identify their mistake they would currently not be able to fix it until they finished all of their classifications. If they are going through quite a lot of variables, then they might also forget to go back and fix the mistake at the end.

It would be useful if there was some sort of functionality that would allow the user to go back to the previous selection and redo their classification. One approach could be if the user enters a specific term instead of the next classification (e.g. "BACK" or "UNDO") then it would go back to the previous variable for them to redo it.

[RayStick]

+1 from Nida (she also requested this)