add a block_group_id column to output data

[bricegnichols]

I tried to add this, but an extra 0 is always injected between the state and tract values, so it still needs to be added in Excel after the acs_combined file is produced.

[stefancoe]

@bricegnichols This might be ok if we can easily construct this same ID on the other files (PUMs & Geocrosswalk) that use block group. I'll take a look.