Subfamily entity

[Per Census](https://www.census.gov/programs-surveys/cps/technical-documentation/subject-definitions.html#subfamily):
>A subfamily is a married couple with or without children, or a single parent with one or more own never-married children under 18 years old.

This is a useful concept because it doesn't require relationship pointers. It can contain at most one married couple, so if a PolicyEngine user says they're married, we can assign them an `is_married` flag and properly assign them marriage-dependent benefits like SSI (see #594). In effect, PolicyEngine assumes one subfamily per tax unit per family per SPM unit per household.