FIX: make pad length a variable in formatted print statement in grouper.py

[sbillinge]

@sbillinge didn't have time to figure out how to incorporate a variable into the formatted print in grouper.py so the pad length of the string (for two-column printing) is hard coded. This needs to be fixed.

[sbillinge]

I am building using an environment with Python 3.5. We may need to make a dependency of python >3.6 to use the full power of f-strings to solve this properly. We could also have a backwards compatible version that will run on python <3.6 but with less desirable output.