Create mapping/unmapping functions for storage candidates

[danielolsen]

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• Is your feature request essential for your project?

Describe the workflow you want to enable

I wish we had a function to create indices for storage candidates, and an easy way to interpret them from the results when applying Grid upgrades. This would be similar to what we did for plant/branch indices in #94

Describe your proposed implementation

Once we can prompt the user for storage expansion candidates (see #69), we will pass this information to a function that creates indices for them. Right now, I'm thinking the bus ID will be most meaningful for extracting results, so e.g. a hypothetical storage at bus 123 would have an ID s123i (s for 'storage', i for investment [?] [hypothetical]).

Then, we will modify the existing recover_plant_indices function to, within the results, separate the built power capacity for storage devices from the built power capacity for traditional generators (both of these are within the BuildGen[GEN_INDEX, BUILD_YEAR] pattern). Or, we can identify storage indices from the results via the BuildStorageEnergy[STORAGE_INDEX, BUILD_YEAR] pattern (storage indices are a subset of generation indices, but only storage indices end up in the BuildStorageEnergy results).

Once we have these, we can use the built storage power/energy plus the input parameters (e.g. efficiency) to reconstruct all information needed for storage within a Grid object.

[danielolsen]

Closed via #101 and #102.