Refactor and improve AEI rebate notebook

- Add SIMULATION_YEAR constant (2026) for clarity
- Modularize code with single calculate_rebate_base_statistics() function
- Function handles both household and tax_unit calculations via parameter
- Add year to all table titles and print statements
- Improve ratio column calculation (programmatic, applies to all rows)
- Use constants for official statistics (CENSUS_HOUSEHOLDS_2023, IRS_RETURNS_2022)
- Always pass explicit year to sim.calculate() calls
- Clean up duplicate code between household and tax unit calculations

Author: MaxGhenis

test_current_values.py

@@ -0,0 +1,14 @@
+# Save current values before refactoring
+print("Recording current values before refactoring...")
+print()
+print("Expected Household Results:")
+print("  Total CA households: 14.2M")
+print("  Households with rebate: 3.3M (23%)")
+print("  Average rebate base: $18,303")
+print("  Total rebate base: $60.5B")
+print()
+print("Expected Tax Unit Results:")
+print("  Total CA tax units: 21.3M")
+print("  Tax units with rebate: 8.9M (42%)")
+print("  Average rebate base: $16,455")
+print("  Total rebate base: $146.5B")