Allow fuel price changes to drive fuel switching in new building components

[jrissman]

Allow changes in the fuel price caused by policies (like carbon tax, fuel taxes, and fuel price deregulation) to change the building owners' choices about which type of fuel newly-purchased building components should use.

This is the buildings sector equivalent of issue #9 (which adds price-driven fuel switching in the industry sector).

Fuel type selection could either be handled by an ALLOCATE AVAILABLE function with appropriately-defined bell curves, or it could be handled by a table of elasticities that specifies the elasticity of shifting to each fuel from each other fuel with respect to the change in the difference in price between those two fuels. (We don’t want to handle only shifts to lower-carbon fuels, to cover the case where someone puts a tax on natural gas but not on fuel oil, for example.)