Add WIC category for multiples

Per a footnote in https://fns-prod.azureedge.net/sites/default/files/resource-files/WICPC2018FoodPackage-1.pdf#page=14:
>Women pregnant with two or more fetuses, women participants partially (mostly) breastfeeding multiple infants from the same pregnancy, and pregnant women who are also partially (mostly) breastfeeding singleton infants receive Food Package VII. Women fully breastfeeding multiple infants from the same pregnancy receive 1.5 times the MMA for supplemental foods.

Currently we assign mothers a single value, with the hierarchy `PREGNANT > POSTPARTUM > BREASTFEEDING`