Price/demand equilibration

[smmaurer]

In some cases we need to intervene in the choice simulation process, for example checking the aggregate attractiveness of neighborhoods and raising their prices until demand falls below supply everywhere. I think this could be done through a separate module that feeds its final parameters into the "lottery choice" module.

• https://github.com/UDST/urbansim/blob/master/urbansim/models/supplydemand.py
• https://github.com/UDST/urbansim/issues/110