Context.agents(...) API should return a consistent type - list or iterable

[etatara]

Context.agents(...)

https://github.com/Repast/repast4py/blob/master/src/repast4py/context.py#L322

returns either a list or an iterable function depending on usage, which might be confusing, since using the returned list or iterable can have different implications depending on how it’s handled, e.g. multiple iters over a list is OK, but only works once on an iterable.