Add capacity payments revenue option to LPF, AEP, SL financial models

Capacity Payments are available as a source of revenue for SAM's Merchant Plant and PPA Single Owner model, but not for the partnership flip and sale leaseback models.

For some projects, such as PURPA <20 MW projects in California, projects can earn revenue from both an energy price in $/kWh (with TOD multipliers in California's case) and a capacity payment in $/kW, as described in this Standard Offer Contract (SOC) from Southern California Edison as mandated by the CPUC: https://www.sce.com/procurement/solicitations/purpa-soc.

Although the ITC is scheduled to phase out, based on current law, it will not do so until after 2024 (https://www.seia.org/initiatives/solar-investment-tax-credit-itc), so partnership flip and sale leaseback structures still have a role to play.

Related issue: https://github.com/NREL/SAM/issues/205