Update `get_generator_marginal_costs` function to allow for links

[trevorb1]

Feature Request

For sector coupling studies, OCGT and CCGT are treated as links. It will be good to update get_generator_marginal_costs to return generators and links based on a carrier level to account for this!

Suggested Solution

No response

Additional Info

https://github.com/PyPSA/pypsa-usa/blob/674f95c2546578610680a61eeaf5b6db932783d9/workflow/scripts/summary.py#L275-L295