Weird magic number in factored-customer/../DefaultUtilityOptimizer

[jecollins]

For population models, the chunk size for tariff evaluation (the smallest number that can be subscribed to an individual tariff) is set to max(1, population / 1000). That 1000 is not configurable, and there's no justification for it. Better to have chunk size a configurable value, particularly since the grid-model integration will need to be able to set it independently.

See DefaultUtilityOptimizer line 93 in the 1.8 version.