Add a scale argument to plot

[erikcs]

It is useful to relate the policy values in plots to problem specific aggregates, for example, in Figure 5 here we show estimates of intervening on n~2500 units. It's possible to achieve this by just scaling your evaluation scores and costs when calling into maq, but a more direct interface would be nicer.

A natural way to do so would be to delegate this choice to plot, e.g,

plot(qini, scale = 2500, ...)

Would plot the policy values and costs multiplied by 2500.

If you want to read off policy values using average_gain you'd have to do the scaling yourself:

2500 * average_gain(qini, spend = something)

(just tagging @swager in case you have any other opinions on this).