Add JRF's Minimum Income Standards

[MaxGhenis]

These are defined per year and household type: https://www.lboro.ac.uk/media/wwwlboroacuk/content/crsp/downloads/mis2020/Annual%20Budgets.xlsx

@nikhilwoodruff feel free to dismiss this as out of scope if the idea is for openfisca-uk to only have government information. This metric contains some controversial aspects, such as setting higher thresholds for working-age individuals than pensioners because they spend more on clothing, but it's a major research initiative for a major think tank.

[nikhilwoodruff]

No I think it's a great idea, thanks for spotting. I don't see any real reason to restrict to government-only financial computations, and I think it makes sense to have these in openfisca-uk.