[FEATURE]: Create central constants file

[trevorb1]

Idea

With each sub-directory of scripts (ie. powerplant, storage, transmission, ect...) we have a constants file. While some constants are specific to these folders, some are more general (ie. tagging renewable, clean, fossil techs). We should really add these to a central constants file.

Is your feature related to a bug?

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Suggested Soltuion

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Additional Info

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