Please describe the problem you are trying to solve.

Watchpoints currently do not include initial data. There are two reasons why initial data is useful:

1. Plotting: especially when comparing watchpoints on different time grids. If the initial data is missing this looks odd.
2. Debugging: initial data has an influence on the final result and the effect of setting initialize="true"/"false" would be easier to spot if watchpoints include initial data.

Describe the solution you propose.
I will add a corresponding PR in a moment. Important: Adding initial data to watchpoints would change the behavior of preCICE and therefore be a breaking change. For adding this feature in v3 we have to make it configurable and set the default to not writing initial data.

Additional context
While working on #2172 I observed a major influence of setting initialize="true"/"false"on the results (I will provide a detailed update in #2172 later). Having initial data in the watchpoint would be useful for debugging in this case.