Compute top-level net income impacts (for decile chart) independently of deciles

[MaxGhenis]

Currently these are averaged across deciles, which results in some reforms having different directional impacts between relative and absolute, e.g. this.

Let's instead compute these as total aggregates, i.e.

• Absolute = total change to net income in £/$ divided by number of households
• Relative = absolute / total baseline net income

Then we should change the client to read:

• [Reform] would {increase, decrease} total net income by $x per household
• [Reform] would {increase, decrease} total net income by y%

[nikhilwoodruff]

Fixed by #199 (mostly on the client side)

[MaxGhenis]

Reopening as a Twitter thread raised a question of how this is computed. For instance, the average of deciles is $250 here, not $201. Is that because deciles are weighted to equal population, making the average across deciles differ from the total average? If so, should we consider changing to per-person?