Vary school days by state

We currently have one number for school days per year for computing school meal subsidies.

The Education Department's National Center for Education Statistics shows the [minimum school days by state](https://nces.ed.gov/programs/statereform/tab1_1-2020.asp). This is concentrated at 180, but ranges from 160 to 185. We'd probably want to look for another source with the average rather than minimum.