fixed InteractionHistogram::operator+()

The code approval procedure is described in the code approval procedure wiki

• The MR is without WIP (work in progress) status
• Make sure the most recent CI jobs (config, quality, build, tests) all run fine with no failures
  • if "check clang-format" failed: the code contributor has to run ./do-clang-format.py --apply eventually with the --all option
  • if "check copyright" failed the code contributor has to run./do-copyright.py --add=20xy
• Also run all the extra optional jobs "coverage", "release-clang-8", "release-u-18.04" and make sure no problems occur
• Check in the "coverage" job output that the coverage did not decreases (!). It should always stay, or increase. If it decreased --> ask contributor to add further unit tests, and check coverage report.
• On the MR page, open the "Open in Web IDE" tool
  • Check if the provided solution solves the Issue, discuss on gitlab
  • Check that all changes are actually related to the issue
  • There are no debug statements left, not even commented out
  • Check all changes for coding rules and guidelines
• When all above is done
  • Add label "Code Review Finished"