Make code changes following data update

[zz-hh-aa]

• [ ] Private rent can read values according to house size (by bedrooms) now; anything above 4 bedrooms uses 4br_monthly_mean_rent
• [ ] pricespaid renamed to price_paid to follow snake case
• [ ] using pence per kwh instead of house average, so can remove some bills math
• [ ] itl_lookup renamed to postcode_lad_itl3_lookup

Blocked by data update