Unit-level random seeds

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 (#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)

Start by tracing how simulation and sample seeds are created and passed into outage draws, then inspect how system specification data is represented for unit-level seed definitions. Done means adding stable unit seeds so adding or removing a unit does not change other units' outage profiles, while preserving reproducibility across runs and samples.