Add Container to Collection Drehimpulserhaltung bei ändernder Umlaufzeit
About containers
The idea of containers is that you can merge for example exercises 1, 2 and 3 as sub-exercises a, b and c of one single exercise with a new title, the container title.
If you add a container, the exercises are treated as sub-exercises with the sub-exercise layout you specified in the layout you choose when building the PDF.
