Set marginal costs for heat links

Deals with #942

Before merging into `dev`-branch, please make sure that

[x] the CHANGELOG.rst was updated.
[x] new and adjusted code is formated using black and isort.
[x] the Dataset-version is updated when existing datasets are adjusted.
[x] the branch was merged into the continuous-integration/run-everything-over-the-weekend-branch.
[x] the workflow is running successful in test mode.
[x] the workflow is running successful in Everything mode.