support proportional membership for weighting

[bstabler]

We're not planning to do this right now, but we don't want to forgot this idea for the future if needed.

In addition to tagging households/persons as T/F membership for attributes, allow for partial membership. For example, instead of is HH size 1, 2, 3 or 4, specify the HH is 80% likely size 1, 15% size 2, 5% size 3, 0% size 4+.