Transform and populate census tract

[pratishta]

This ticket combines step 7 (transform the data from the source tables into the model tables) and 8 (copy the model tables into the zoning-api's target tables) of data-flow.

Acceptance criteria

• Add a sql file for census tracts under pg/model-transform that transforms the data as needed and populates the model table census_tract
• Add a sql file for census tracts under pg/target-populate to create an updated census_tract from the csv created in the step above