Adopt consistent approach to looking up TANF values based on household size

In some states (e.g. TX) we use a scale parameter to capture the 5+ people category, others (e.g. NJ) we cap at the maximum then look up with smart indexing.

Start by comparing the TANF household-size lookup implementations for Texas, which uses a scale parameter, and New Jersey, which caps at the maximum and uses smart indexing. Determine which approach should be standard, then verify that the affected state rules produce consistent results for households of five or more people.

Written by the indexing model from the issue text.