Vector ssb [Don't merge]

This PR depends on https://github.com/OpenSourcePolicyCenter/Tax-Calculator/pull/52, so it looks messier than it is. This [commit](https://github.com/talumbau/Tax-Calculator/commit/13887ecba4d08f44a1ea832533a8a6b86c632128) is relevant to the discussion.
To confirm, when we use dataframe_vectorize (or ex_nmba.py) we don't need to declare a global here, right?