Use more detailed capital gains distributions for top income centiles

[MaxGhenis]

We currently impute capital gains using only Table A1 from Advani and Summer (2020): p5/10/25/50/75/90/95 by income centile. Table A2 shows all 5p quantiles, plus p95/96/97/98/99 which are especially important.

[MaxGhenis]

Closing as migrated. Detailed capital-gains source-stage work is tracked in PolicyEngine/populace#145 and #148.