Keep families together callback

[dtburk]

Copied from mnpopcenter/hipread#5

Jul 10, 2018 @gergness:

Would be nice to be able to get chunks that keep households together. Need to think through how this would work for arbitrary record type configurations though.

(It may be a long time before IPUMS projects get their act together regarding rectype ID variables, so it'd be nice if this worked without them, possibly building on Mark's PR to ipumsr)