Customize output_times to allow network output for desired time ranges

[PeterCarragher]

Construct.h get_output_timeperiods() returns a vector with just the 0 timestep (at least for the friedkin model).
To be able to output networks for each timestep, should_process() could use a min / max timestep, which defaults to all timesteps but can be customized to the desired range.
Parameters could be passed at model initialization when Output class is instantiated.