Improve DBF extraction functionality

[e-belfer]

We've added FERC Form 2, 6 and 60 DBF data to the DBF extraction process in #1984. Let's make some quality improvements to this process by explaining size differences between FERC 1 and the newly-extracted datasets, and by building in foreign key constraint checks to ensure there are no unexpected big surprises in the raw data.

• Check foreign key constraints in DBF derived FERC SQLite DBs #2674
• Investigate why FERC 2 and FERC 6 DBs are so much smaller relative to FERC 1.