Add OBR long-term economic determinants

[nikhilwoodruff]

Summary

This PR implements long-term economic determinants alongside various improvements and bug fixes to the PolicyEngine UK codebase.

Changes

Economic modelling

• Added long-term economic determinants for improved forecasting and analysis
• Integrated OBR labour supply response elasticities for more accurate economic modelling

Bug fixes

• Fixed rent and council tax variables that were causing errors in individual simulations (these variables are now obsolete as uprating is handled in /data)
• Increased error margins in extended childcare entitlement tests to address minor calculation differences

New features

• Added simulation comparison utility (compare method) for analysing differences between baseline and reformed simulations
• Added variable dependency fetching (get_variable_dependencies method) for understanding calculation relationships

Documentation

• Updated simulation documentation to include new comparison and dependency utilities

Related issues

Closes #1306 (rent/council tax bug fix)
Closes #1307 (simulation utilities)
Closes #1304

Testing

All tests pass with updated error margins for extended childcare entitlement calculations.