Sam 391 ssc 825 merchant plant forecast upgrades

[brtietz]

1. Updates calculations in common.cpp to send a price signal to FOM automated dispatch with the capacity-weighted sum of merchant plant markets (as opposed to the straight sum). Should improve dispatch decisions for merchant plant when running as a percent of generation
2. Corrects these calculations for single-value merchant plant inputs. (these were fixed for the main merchant plant code back in Fixes SAM issue 1004 #823, bringing that fix to this upstream class)
3. The forecast price signals class calculates a cleared capacity value that is passed to FOM automated dispatch
4. FOM automated dispatch can use the new cleared capacity information like an interconnection limit that needs to be charged sometimes.

The current implementation is somewhat weighted to "charge as soon as possible to meet cleared capacity" but seems to meet cleared capacity requirements. I have some ideas on how to back it off and make it more economic if users complain.

Closes #825
Also closes NREL/SAM#391

Test file:
merchant_plant_dispatch_test_single_cc.zip

Note that markets can be all based on percent of generation, all based on cleared capacity, or a combination! This should handle all of the above, but requires large batteries and a small % of generation to make it through to the end.

It might be nice for debugging to make some of the merchant plant errors optional to assist with sizing and debugging for these types of applications. Probably not a feature for this release though.