Change the interpretation of zoning_heights in feasibility model

[hanase]

The UDST feasibility model determines the max FAR for mix-use parcels using the zoning_heights table as follows:

1. FAR from the heights column is computed (max_far_from_heights).
2. If the table contains the column max_dua, it calculates the corresponding FAR (max_far_from_dua) and takes the minimum of max_far, max_far_from_heights, and max_far_from_dua.
3. Otherwise, it takes the minimum of max_far and max_far_from_heights.

The last two steps are implemented in the _min_max_fars function.

Our interpretation of the table is that max_far limits the non-residential part whereas max_dua limits the residential part of the mixed development. Thus, taking the minimum over these is wrong.

[hanase]

We now have a child class that overwrites the _min_max_fars function. Step 2. above is replaced by taking the sum of max_far_from_dua and max_far capped by the max_far_from_heights. In Step 3., max_far_from_heights is only considered if max_far is not given.

[hanase]

@jensenmj Mike, given Beccah's notes on the FLU columns, how do you think this should be changed?