Merchant plant to battery forecast input sums inappropriately

[brtietz]

Describe the bug
If you provide a price of $20/MWh to just the energy market in the merchant plant, the battery will receive a price for that timestep of $20/MWh. If you provide that same price to all 5 markets, those values will be summed, and the battery will assume it's receiving $100/MWh. While the percentage price differences remain the same between steps, the magnitude matters when comparing to the cost of cycle degradation.

To Reproduce
Steps to reproduce the behavior:

1. Run the attached SAM file: aligned price series.zip
2. Compare the "power price for battery dispatch" between the all energy market case and the all markets case.

Expected behavior
If we're only going to provide one price signal to battery dispatch, it should be scaled appropriately to the number of markets active. Discuss whether we can use a simple average or a weighted average, and if this needs to factor into the solution for NREL/SAM#181

[brtietz]

Easiest valid approach: weighted average. This has potential issues with cleared capacity as a MW value, where the battery might not decide to dispatch & fail to meet the cleared capacity. See if 181 can address this or if we need to do something else.

Battery price computation in a difference cmod: break this out of the battery code since it potentially contributes to financial model modularity.