feat: added adjusted and retrofitted gas bills

[zz-hh-aa]

What does this PR do?

• Adds gasBillAdjustedYearly and gasBillRetrofitYearly to the Household class
  • They are now included in the Household constructor, and also have their own calculation functions calculateGasBillAdjustedYearly and calculateGasBillRetrofitYearly
• Updates the Lifetime class to include gasBillAdjustedYearly and gasBillRetrofitYearly, inflating their values year on year
  • interface LifetimeParams is updated, as is interface LifetimeData and the calculateLifetime function itself
• Passes the gas bills to the graphs
  • gasBillAdjustedYearly to LifetimeMarketPurchaseWrapper and LifetimeMarketRentWrapper
  • gasBillRetrofitYearly to LifetimeFairholdLandPurchaseWrapper and LifetimeFairholdLandRentWrapper

Why?

• The previous gasBillYearly value we were getting was one static figure for the average house, assuming heating demand of 13,600 kwh. Since the calculator needs to be able to account for different house sizes, types and insulation, one value wasn't sufficient.
• gasBillAdjustedYearly takes the price per kwh and uses the gasBillAverageYearly (retried from database), property.size and property.houseType to estimate a more accurate version of bills for the existing property
  • gasBillRetrofitYearly does the same but with lower kwh demands per m2

Closes #26

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[zz-hh-aa]

Thanks @gabrielegranello! I guess there's an open question about how we want to document sources--does it make sense to put links in the README instead of in the code? I think comments might get messy with a bunch of URLs stuck in. Maybe this is something we can talk about this Friday!

[zz-hh-aa]

Made a mess of this in the rebase / merge conflict fixing; since we have new data soon and needed to replace the math anyway, I'm just going to close this PR.