Add a reference category for the cost-effectiveness plane for PSA results

[mbkauf]

Center the plane on (0,0) and create an incremental cost-effectiveness plane if a reference strategy is provided. This changes will be made directly to the plot.psa() function.

[mbkauf]

@feralaes @evaenns @ppehli @krijkamp

The function add_common_aes() uses the default ggplot colors for a discrete scale. This means that which colors correspond to which strategy are dependent on the number of strategies on the plot. For the purposes of this new functionality, the colors for each strategy change when the reference strategy is removed from the plot. I've attached examples using the PSA vignette Is this something that we are okay with?