Write a price function for developer model

[hanase]

In the feasibility part of the developer model, price for each use (i.e. building type) is computed on each developable parcel. The corresponding function is passed via the parcel_price_callback argument. It should give yearly rent per sqft.

Currently it is simply the land value implemented in parcel_price_placeholder. Replace the placeholder function with something useful.

[hanase]

I replaced the placeholder function with a function that applies coefficients from our expected sales price model. It is implemented in parcel_sales_price_sqft_func.