iebaltab: add numbers to all columns

[luizaandrade]

@mariaruth suggested we add an option to display column numbers for all columns, not only the group means, so they can be more easily referred in the text. It would look like this:

[kbjarkefur]

I like this and should not be too difficult, but I think it should be part of the decision to re-write iebaltab where we separate the code that do the calculation from the code that does the output.

[luizaandrade]

Yes, I agree. I'm hoping to have some time to work on that after the summer school.