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.
I did a "minimally invasive surgery" here to show this example. In the final version, I would advocate that this global declaration go away, but nothing here would force that to happen. Removing it would be motivated by a desire to be clear on when variables enter the global namespace. This global declaration is confusing because it puts the name `c02500` into the module-level namespace the first time the function is called. That's pretty unorthodox and makes these functions sensitive to the order they are called.