Switch to anchored Supplemental Poverty Measure

The Supplemental Poverty Measure is a partly relative measure, as it bases thresholds on the 33rd percentile of a certain consumption basket.

Columbia University has defined an "anchored" SPM tied to 2012 consumption levels, making it a true absolute poverty measure. Especially as we forecast poverty into the future, we'll want to similarly anchor for consistency with UK measures and to distinguish poverty from inequality.