Add flake8-print

[PGijsbers]

We do not want print statements in our code (use logging).
Flake8-print is a plugin for detection of print statements, and can be added to our style guide enforcer.
See flake8-print.
It should be added to the pre-commit routine.

[PGijsbers]

Listed as "good first issue" because it is completely unrelated to our actual code base, so in that sense it has a lower barrier to entry.