Unit-level random seeds

[GordStephen]

Define unit-level random seeds to enable adding or removing resources without impacting the random outages associated with other resources. Currently, if a new unit is added, the simulation+sample seed will be used for the additional outage draws, changing which random numbers are associated with which units and thus the outage profiles of all units.

This would add a third level of seeding to a simulation:

1. Simulation seed: primary random seed for the entire simulation. Provided (or generated automatically) when the simulation is initiated.
2. Sample seed: secondary seed assigned to each sample from 1:nsamples. Generated automatically when makesamples sends simulation jobs to the threaded coroutines. Providing these directly (Allow choosing specific sample seeds #36) would allow recreating specific random outage conditions
3. Unit seed: new, tertiary seed assigned to each unit. Fixed across runs and samples and defined in the system specification data (e.g. .pras files)