Implement logic for capital gains taxation

[MaxGhenis]

This won't have a real effect because the FRS doesn't have capital gains data (per #11 (comment)), but could implement to have it.

Here are the rates: https://www.gov.uk/capital-gains-tax/rates

With stamp duties and inheritance taxes, it represents 5% of tax revenue (1/9 of income tax + NI): https://www.ifs.org.uk/publications/9178

[MaxGhenis]

Will SPI enable us to impute this? Hate to cite Statista, but apparently it's 10bn pounds/year: https://www.statista.com/statistics/284309/united-kingdom-hmrc-tax-receipts-capital-gains-tax/

And since it's only taxed at 20% for high earners, and represents a disproportionate income share among that group, it could be a significant part of the additional progressivity from applying the SPI adjustment.

[MaxGhenis]

Shoot!

Capital Gains arising from the disposal of assets are subject to Capital Gains Tax (CGT) and are not treated as income for Income Tax purposes, so gains from the disposal of assets are not included in the SPI.

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/895281/SPI_National_Statistics_Supporting_Documentation_2017_to_2018.pdf

Is there any dataset with capital gains?

Edit: let's discuss data in nikhilwoodruff/frs#12.