Minor Power Plant Costing Fixes

[bpaul4]

Fixes

Minor modifications to power plant costing.

Changes proposed in this PR:

• correct units on a couple of terms
• set additional_cost_of_electricity to initialize at 0, add details to docstring
• fix transport_cost units and handling

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 76.35%. Comparing base (3c209b9) to head (1f828e4). Report is 1 commits behind head on main.

Files	Patch %	Lines
...ra/power_generation/costing/power_plant_capcost.py	0.00%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1418 +/- ## ========================================== - Coverage 76.36% 76.35% -0.01% ========================================== Files 394 394 Lines 64953 64953 Branches 14404 14404 ========================================== - Hits 49601 49598 -3 - Misses 12793 12797 +4 + Partials 2559 2558 -1 ```

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