Chunk by census block groups

[lxbarth]

Use census block groups for more granular chunks to upload:

• http://dcatlas.dcgis.dc.gov/catalog/results.asp?pretype=All&alpha=C

/cc @mikelmaron

Tracts

This is what we're using now to chunk up data.

Block groups

Update: edited to use block groups.

[andinocl]

+1 for block groups for initial import (allows more hands, closer bathroom breaks).

[lxbarth]

@andinocl agreed.