Add existing PHS in add_electricity

[WeiAi-Energy]

Feature Request

We've already have existing power plants and existing battery storage in add_electricity. But for PHS, it seems like we can only model its green-field expansion. #337

Suggested Solution

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Additional Info

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[WeiAi-Energy]

Here is the dataset that REEDS used to model existing PHS: https://github.com/NREL/ReEDS-2.0/blob/main/inputs/capacity_exogenous/ReEDS_generator_database_final_EIA-NEMS.csv. It only brings with power capacity and I'm not able to find any source for energy capacity at plant-level, so maybe we have to approximate the energy capacity for each plant based on the nation-level number of 553 GWh: https://www.energy.gov/eere/water/hydropower-market-reports.