Code blocks are temporarily removed

[schalkdaniel]

In #28 we removed the code exercises. To bring them back, one has to remove the comments from a code block.

All coding exercises are collected in the exercises folder. To bring the exercises back do the following:

1. Go into the exercises folder and look which exercise you want to include e.g. exc_05_06_05.R
2. Open the associated chapter chapters/chapter05_06_xxx.md
3. Remove the <!-- --> comments of the exercise
4. Voila