Update cp-worker for readability? #141

@bethac07 How do you feel about me re-writing a couple parts of DCP to increase readability?
Basically, I'd love to re-write to use f-strings for the CellProfiler command so it's easier to read/understand.
It'd be nice to bring this part of the code into the land of Python3, but I want to make sure I'm not missing a reason not to do so.