Refactor Agent initialization

[tilacog]

Currently, we require a pair of matching Operator and Network lists to instantiate the Agent object, but that could be simplified by just passing a NetworkSpecification list to its constructor.

Originally posted by @hopeyen in #668 (comment)

It seems that Networks and Operators both came from mapping on NetworkSpecification, so I would like to understand the flow to separately derive Networks and Operators from NetworkSpecification, validate their positions as two arrays, then zip into "NetworkAndOperator". Is it not possible to simply validate NetworkSpecification for its Network and Operator info directly?