Refactor income imputation and remove winter fuel allowance from loss calculations

[nikhilwoodruff]

Fixes #210

This refactors the income imputation logic to be more selective about what gets imputed on the main dataset.

Changes:

• Extracted imputation logic into a reusable impute_over_incomes function
• Now only imputes dividend income on the main dataset (while still imputing all income components on the zero-weight copy)
• Removed winter fuel allowance from loss calculations (both count and total metrics)

This should improve accuracy by being more targeted about which income components need imputation on the full dataset.