Audit SimExec documentation and units

[Mazhar331]

Follow-up from PR #125.

PR #125 is scoped to #76 raw-material OPEX accounting. During the review
follow-up, only the PR-added or PR-touched raw-material helpers and tests were
documented. A broader pass is still needed for pre-existing SimExec.py
functions, class-level documentation, and unit annotations that were not part
of that PR.

Scope:

• Add NumPy-style documentation for pre-existing SimulationExec methods and
  class-level behavior that currently lack docstrings.
• Add or normalize units in docstrings/comments using the bracketed format,
  for example [kg], [mol], [s], [USD], [USD/yr], and [-].
• Audit OPEX/CAPEX/material-accounting time bases so returned quantities state
  whether they are per simulated run, per year, or dimensionless [-].
• Avoid changing calculation behavior unless a separate bug fix is identified.
• Coordinate with Document SimExec labor cost assumptions and references #127 for labor-cost documentation and Add references for SimExec cost constants #128 for cost-constant
  references instead of duplicating those narrower follow-ups.

This issue should not block PR #125 unless a reviewer identifies a specific
raw-material OPEX documentation or unit gap introduced by that PR.