Better LaTeX-Code, trees now look better

[Ooooscar]

I defined the empty tree as a new command (in order to get rid of some redundant LaTeX code).

Only the "Exercises" PDF is changed, but similar ideas can be applied to other files.

Maybe the new commands are better defined in the main TeX template files, in order to avoid errors caused by new commands in different exercises having the same name (and before \begin{document} for better code style).

[ossner]

Thanks for this, makes a lot of sense. It would be great if you could also make the changes to the solutions, so that this is all contained in one PR :)

[Ooooscar]

Yes, I think I will do this after my exams :)