Impute spouse's tax information when filing separately

This is necessary to compute the [NY household credit](https://www.tax.ny.gov/pdf/current_forms/it/it203i.pdf#page=28):

In #897 I'll just assume that `adjusted_gross_income` and `tax_unit_size` are identical to the filer's own return, but leaving this here in case we can find a more accurate method.