Add benefit income variables

[nikhilwoodruff]

The relevant variables are:

• mcaid_ben
• mcare_ben
• ssi_ben
• tanf_ben
• vet_ben
• wic_ben
• snap_ben
• housing_ben
• other_ben

This should be de-prioritised since we're aiming to model many of these anyway.

[MaxGhenis]

I intentionally removed these from openfisca-us 0.19.3 because we'll be doing them differently and the C-TAM software that produces them is no longer reliable. I'd suggest we skip these from openfisca-us-data too for any taxcalc validation, and instead import them straight to openfisca-style variables.