python version of evaluator block

[guruofquality]

Create a more flexible eval() block that ships with the python blocks:

• See https://github.com/pothosware/PothosBlocks/blob/master/event/Evaluator.cpp

[ncorgan]

WIP: https://github.com/pothosware/PothosPython/tree/wip/evaluator