added exercise for understanding of eigenmodes - closer to highline

[PhiSpel]

changed according to @PhiSpel 's comments

Description

514 should afterwards be ready to merge

Checklist

• [x] This pull request is associated to an issue
• [x] Add someone else as reviewer and wait for approval before merging.

[PhiSpel]

@rstrickstrock if you're fine with these changes, approve so I can merge to https://github.com/hbrs-cse/Modellbildung-und-Simulation/tree/513-add-understanding-exercises-to-eigenwerte-balken and merge #513

[rstrickstrock]

I think everything is good, now.