Overhaul mcfgen.py and library_setup.py

[rsdefever]

A few months back I wrote an overhaul to mcfgen.py and library_setup.py. See numbers #32 and #33 for the code and reference. I have refrained from merging the changes into the main repository because I am concerned that the updated scripts have not yet had adequate testing. I propose we move forward with integrating these into the repository but first add pytest-based unit testing on these two scripts. See #83 for further motivation.

This overhaul will also move us to python3 for both scripts. We have now included a python2 deprecation warning with both scripts for a couple of releases but I nonetheless propose that we continue to publish the python2 compatible versions of the scripts at least for another release or two, at which time they would be removed.