Implement some performance improvements

[fwitte]

This transitioned from the idea of implementing line search to performance improvements somehow. Specifically caching sectionedheatexchanger temperatures and caching CoolProp calls in postprocessing.

• Implement unit tests for the changes
• Update the documentation
• For a new feature: Implement an example and a system test
• Update the changelog at docs/whats_new/*
• (Optional) update the CITATION.cff file, if you want to be included in
  the citable contributor list