Update city parcels based on new agency clarifications

[michaelkirk]

(extracted from #16)

We want to identify unused city parcels, but our parcel dataset is county wide.

In the county parcel dataset there is an "agency" field which can help determine if the parcel is city owned.

Over in #17 @juansvas has kindly deciphered exactly which agencies are city-owned. Using that data we should be able to regenerate our parcel dataset for a more complete listing of vacant parcels in the city.

Ideally this would be done by writing a script that can be re-run if our input dataset is updated.