Add documentation/feature to extract whole system cost

[GabrielKS]

Relayed by @claytonpbarrows, there is a request to be able to easily extract the total operational cost of a simulation — not quite the same thing as the objective function value. There are two ways this could be done:

1. Documentation of existing feature: we already have the calc_total_cost metric, which operates on individual generators, so one could just do something like:

calc_total_cost(make_selector(Generator; groupby = :all), sys)

. I should make sure there aren't any edge cases; if something like this actually works it seems like we should just go this route and put it in a tutorial somewhere.
2. Addition of new feature: there could be a new system-level metric:

calc_system_cost(sys)

that bakes in any edge cases (and would likely be a bit more performant, not that performance should be a huge issue).

Once #24 is merged and released (this week, probably), I'll work on this.