Add existing storage devices when preparing Switch inputs

[danielolsen]

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• Is your feature request essential for your project?

Describe the workflow you want to enable

I wish a Grid object containing energy storage could be used to create Switch inputs, and the existing energy storage would be interpreted as a fixed investment like other existing generators. This was brought up in #101 but considered out of scope of that PR.

Describe your proposed implementation

We probably want to change the call signatures of build_generation_projects_info, build_gen_build_costs, and build_gen_build_predetermined to take not only the plant dataframe, but a whole Grid, so that the information about existing storage devices can be added the the eventual output of these CSVs as well.