Added Python evaluator block

[ncorgan]

• Closes python version of evaluator block #20

Differences from /blocks/evaluator

• Takes in list of Python modules to import before evaluating expression
• "Global variables" are labelled "local variables", as global and local have specific definitions in Python
• As of now, multi-argument output is unsupported

@guruofquality Could you test the attached file in PothosFlow (changed extension so Github accepts it)? My unit test passes fine, but I'm getting hangs in PothosFlow changing the slider, pausing, reloading the registry, etc, while running this.

evalpothos.txt

[guruofquality]

@guruofquality Could you test the attached file in PothosFlow (changed extension so Github accepts it)? My unit test passes fine, but I'm getting hangs in PothosFlow changing the slider, pausing, reloading the registry, etc, while running this.

setattr(Evaluator, setterName, partial(self.__setter, name))

I think it ends up holding a reference to itself, if you can pass a different function to partial with maybe a weakref to the self object that should work.