Add imbalance settlement calculation

[jdhoffa]

Penalize deviation from day-ahead schedule with imbalance price. The simulation should print the final "cost" of the imbalance.

[jdhoffa]

Acceptance criteria proposal:

• Define formula:
  • imbalance_kw = feeder_kw - target_kw
  • interval_cost = price_per_kwh * abs(imbalance_kw) * dt_hours
  • total_cost = sum(interval_cost)
• Final report prints total imbalance cost
• Optional per-step cost in telemetry
• Unit tests for units/sign and known-case totals