Enhance thermal test documentation

[danielfeismann]

• [ ] find test cases for thermal grids and HP etc. in textbook
• [ ] update test after #/all-tbw is merged
• [ ] if there aren't test cases from textbooks enhance the documentation of tests

I would really love to have better commentary within most tests that were introduced here. Specifically, wherever we calculate power/heat etc., there should be some explanation on how we arrive there.

I am not sure though whether we should provide these remarks in here, since implementation and tests will change again with other PRs. So maybe we should just invest the work there, not here.

In the long term, it'd be great to have test cases like for PvModel where functions are tested with textbook examples etc.

Originally posted by @sebastian-peter in https://github.com/ie3-institute/simona/pull/316#pullrequestreview-1759720386