Add exception handling for SSAs

[dfsnow]

SSAs are a tool that add a fixed tax rate district to an area to provide services. Unlike other taxing districts, the tax rate of an SSA does not vary. In the data, the tax rate is kept fixed by varying the levy of the SSA as the tax base grows or shrinks.

For historical bills, PTAXSIM works fine as is. However, when calculating counterfactuals, SSAs can become a problem since any change in the base must be reflected in the levy in order to keep the SSA rate constant (and the counterfactual accurate). As such, we should add a warning or a helper function specifically to deal with SSAs.