Add Objective Discounting

[Ethan-Russell]

@sallyrobson from our conversation today, we want an annual objective expression, so that we can sum with a (custom) discounting function.

I'm imagining providing something like the in the config file:

discounting:
  type:  Weighted
  rate:  0.03

[Ethan-Russell]

This is only relevant for multi-year simulations. Hold off on it for now.

[sallyrobson]

Dan brought up that we currently don't represent the fact that different stakeholders/participants (gov, generators, etc.) in our model might have different discount rates. This is something that isn't (at least widely) done and so could be a contribution to the field of policy analysis in general. It might be important for overall benefits because it will change the relative impacts of changing profits for different groups. We should talk about this when we get to objective discounting.

[Ethan-Russell]

@domeshek-rff @NicholasHRoy

[Ethan-Russell]

From Dan on 5/17:

We may need different discount rates for different types of markets.