Add additional pricing strategies - weighted average full cost and weighted average marginal cost

[ahawkes]

Is your feature request related to a problem? Please describe

At the moment MUSE2 has 4 pricing strategies. These are:

1. shadow: Prices provided by the dispatch optimisation solver (commodity balance dual solution values). These are useful system value signals, but are arguably not reflective of real-world prices.
2. scarcity: These are the shadow prices with the impact of scarcity removed. e.g. where a cheaper generator is constrained off due to insufficient capacity, prices rise according to the value of having more capacity of that process.
3. marginal: These are our own calculated marginal prices for each asset (and candidate assets where the commodity isn't present in the previous MSY, but could be introduced in the current MSY). They are similar to shadow prices, but add more real-world realism by avoiding zeros and avoiding scarcity pricing.
4. full: These are our own calculated full-cost prices. They use the marginal prices for process inputs/outputs, but also add a portion of the annualised fixed costs (capex and fixom) in proportion to activity in each time slice. The commodity price is the highest cost process in the time slice.

We need a few more pricing strategies to better reflect real-world observations. These are versions of (3) and (4), but where weighted averages are taken instead of taking the highest cost process as the price.

Describe the solution you'd like

Add two more pricing strategies:

5. marginal-average: Same as marginal, but the commodity price is set as the load-weighted average of all processes contributing in the time slice.
6. full-average: Same as full, but the commodity price is set as the load-weighted average of all processes contributing in the time slice.