Main changes of this PR

Removes warning that tells user to not initialize data of second participant in serial coupling, if waveform order is zero. Initialization actually becomes reasonable in this case, if the user wants to sample at the starting time (we cannot know whether the user will sample or not, because the sampling happens over the API).

This PR also removes the warning to avoid the test Integration/Serial/Time/Implicit/SerialCoupling/ReadWriteScalarDataWithWaveformSamplingZero to trigger the warning. The test is correct and should not trigger any warning that suggests a change (because this will lead to a failure of the test).

Motivation and additional information

This change is motivated by #1422. See this change in the test: 270cc08#diff-52e93df8fe0311c9dcda9c30ccdb04ac3fd7d92fc5f7576ae6b261a40e23fb83L81-R81: At the beginning of the window we now get the data from the last window or initial data, even for zeroth order waveforms. This makes data initialization relevant for all coupling schemes and participants, if a user wants to sample the function at the beginning of the simulation time.

See also comments under #1414 (comment) for further discussion.

Author's checklist

• I used the pre-commit hook to prevent dirty commits and used pre-commit run --all to format old commits.
• I added a changelog file with make changelog if there are user-observable changes since the last release.
• I added a test to cover the proposed changes in our test suite. (not relevant)
• I sticked to C++17 features.
• I sticked to CMake version 3.16.3.
• I squashed / am about to squash all commits that should be seen as one.

Reviewers' checklist

• Does the changelog entry make sense? Is it formatted correctly?
• Do you understand the code changes?