Rename kA to UA and zeta to zeta_d4

[fwitte]

Resolves #979
Resolves #976

This PR will also add a new consistent parameter lmtd for all heat exchanger types which is equal to Q/UA

• 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

[fwitte]

Everything changed in src tests still all pass (except exerpy related ones), so green light here.

[fwitte]

Last missing piece is a unit test that verifies the conversion of the parameters. Right now the tests implicitly do because all the test still use the old zeta/kA specifications. But this will also be changed, so there must be a test that checks if specification of a deprecated parameter which will be renamed will correctly specify the renamed parameter and its result will correctly be passed back to the deprecated parameter. This is to be checked for

• the value itself
• is_set
• is_var