Remove dupes and adjust coverage in JSON

This does two things:

1. Removes duplicate and incorrect entries. For example, there's a 300MB parcel dump from Minnesota that covers 7 counties. I removed the duplicates, renamed the source file, and added the list of counties to the JSON. Also, there were 3 duplicates that were marked for Utah when they should have just been Wyoming.
2. To support the above, each JSON now more explicitly refers to a single source file and describes its coverage area.