Impute capital gains

[MaxGhenis]

Moving from https://github.com/nikhilwoodruff/frs/issues/12; see also https://github.com/PSLmodels/openfisca-uk/issues/40 which covers implementation of capital gains tax logic.

Neither FRS nor SPI nor WAS microdata includes capital gains, but we could impute them from HMRC statistical tables. Here are some high level stats from 2018-19:

• 256,000 individuals paid 8.8bn tax on 59.3bn capital gains (table 1)
• More than half of gains are from people aged 55 or older (table 6)
• Table 3 has the most important information (far left column is lower limit, left of each pair of columns is individuals in 000s, right is gains in millions):

The Office of Tax Simplification has some additional data. For example, the below chart shows that people bunch capital gains below the 12,300 threshold; while the report includes data for some charts, it omits it from this one.

[MaxGhenis]

@nikhilwoodruff found that Landman Economics imputes within categories proportionally to capital holdings.