mainly adding functionality to add supdem to urbansim_defaults

the lcm_simulate function is now becoming pretty involved - there's quite a bit of code required to get the logic of supdem correct.  I will document this further in a response to this issue https://github.com/synthicity/urbansim/issues/110

This commit also adds a few variables to urbansim_defaults which compute total zoned_capacity and remaining zoned capacity (i.e. removing currently developed units)