Document SimExec labor cost assumptions and references

[Mazhar331]

Follow-up from PR #125 and review comment:
#125 (comment)

PR #125 is scoped to raw-material OPEX accounting, so the new GetLabor
documentation added during review was moved out of that branch.

Scope:

• Add NumPy-style documentation for SimulationExec.GetLabor.
• Clarify the default wage 35 [USD/h], operating weeks 48 [week/yr],
  and the worker/shift multipliers used in the current formula.
• Resolve whether the labor-cost TODO should be interpreted as hourly [1/h]
  or per-shift [1/shift] accounting.
• Add original references or explicit assumption notes for labor constants.
• Preserve the bracketed unit convention in comments and documentation.