Generate output objects of execution edges

[bbartley]

Output tokens that are generated by an action execution should be typed based on their Parameter.type. Currently, however, output tokens are generated as literal strings by default, even when the expected output is an object type. While there are some special cases for the EmptyContainer and PlateCoordinates primitives that do generate the right type of output objects, we need a more generalized handler that can generate output objects of the appropriate type.

Output token generation is currently handled by primitive_compute_output method in execution_engine.py