Main changes of this PR

Add API function initializeWriteScalarData(writeData, vertexIDs, dataID) which allows to set value of Waveform at starting time of the simulation. This is an optional API call.

Motivation and additional information

Currently we can only use constant interpolation in the first window, since only one data point is available from writeScalarData. This data is associated with the end of the window. Offering an API call initializeWriteScalarData which must be called before initializeData allows us to provide data associated with the beginning of the first window. Together with the data provided via writeScalarData this allows linear interpolation already in the first window.

Note that for the second and following windows the data from the previous window will be used to define the linear interpolation. Therefore, the API call initializeWriteScalarData should only change the behavior in the first window.

Todos

• merge Store from/toWaveform in DataContext #1
• extend tests to this new API function and make sure that linear interpolation in first window is supported.

Author's checklist

• I added a changelog file with this PR number in docs/changelog/ if there are noteworthy changes.
• I ran tools/formatting/check-format and everything is formatted correctly.
• I sticked to C++14 features.
• I sticked to CMake version 3.10.
• 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?
• (more questions/tasks)