Powerplant data functions

[maartenbrinkerink]

Description

Large overhaul of the powerplant_data script by splitting 'main' in individual functions as it was incredibly difficult to navigate and problem solve. Have not yet separated transmission from the powerplant_data script (next step).

Issue Ticket Number

Documentation

[maartenbrinkerink]

@trevorb1 I've finished the refactoring of the powerplant script which on my end is now functional both standalone from main as well as by running the full workflow. If you have time could you do a review at this point?

Note that I've moved the configurable variables to support the standalone functioning of the set of scripts from main to constants to contain the number of places where users should be looking for custom changes. If you agree with this approach I suggest to apply the same for the demand folder. Let me know!