add Census units w/ topology relations

[ksonda]

@fils and I can work on hosting Census geographies down to the block level in reference.geoconnex.us.

Need to make

• id scheme that accounts for time dependence
• relation scheme that tracks how units across time are the same, split, aggregated, or related only via spatial intersection
• probably need to reorganize reference.geoconnex.us to be based on a postgis db with the data size involved.

[ksonda]

Census 2010 - 2020 relationship files

https://www.census.gov/geographies/reference-files/time-series/geo/relationship-files.html

[ksonda]

Probably need to make a repo similar to https://github.com/dblodgett-usgs/ref_gages expressly for this purpose...

[ksonda]

I'm doing this here now https://github.com/ksonda/geoconnex-census