Add sim_data_generator.py with gen_data() function

Created a new data generator that uses the full supply chain simulation
to generate data in the exact same format as forecast_app/data_generator.py.

Key features:
- gen_data(config) function for easy integration
- Uses complete supply chain simulation with all entities
- Outputs data in forecast_app compatible format
- Supports configurable parameters (seed, years, scenario, etc.)
- Aggregates simulation results into monthly watch-level metrics
- Includes all required fields: demand, production, inventory, costs, profit

The generator can be imported directly:
  from supply_chain_sim import gen_data
  dataset = gen_data(config)