Feasibility Staleness hotfix

[akselx]

This branch is a hotfix for a stale feasiblity.

The issue is that a parcel may be feasible for multiple buildings - and may be assigned a residential building in residential_developer() only to be found feasible for a large retail project later, in retail_developer(). In practice, this joint allocation is unlikely, and should be handled only through explicit mixed use development forms.

Since the feasibility step is run before the first developer model step (residential) in a year's simulation cycle, it predates the placement of a building in the residential developer.

This fix re-runs the feasibility step before each developer model type.

The fix is implemented as:

• additional feasiblity steps identified in baus.py
• a separate filter for the developer model to only consider parcels not already developed by the developer model

Since this branch is built on top of #379 that PR should be merged first.

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