Fix Montana AGI additions/subtraction logic for social security benefits

[martinholmer]

In the 0.759.1 version, the MT addition and subtraction logic for social security benefits is incorrect.

The 2021 Form 2 (page 6) indicates that the amount of federally taxable social security benefits can be adjusted up or down when calculating MT AGI. Here is the form schedule for doing that:

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But the PEUS code incorrectly adds all social security benefits to federal AGI when calculating MT AGI (see parameter file below). This is a particularly glaring error because, for many tax units, some social security benefits are already included in federal AGI. Also, the code does not include the possibility that there could be a social security benefits subtraction when calculating MT AGI.

https://github.com/PolicyEngine/policyengine-us/blob/91f05ce8327f97dfa6d430997ce92467251f1b92/policyengine_us/parameters/gov/states/mt/tax/income/additions/additions.yaml#L1-L15