Merge branch 'cleanup/model' into 'master'

[ex][model] Add link to exercise

See merge request !261

slides/model.md

@@ -199,7 +199,7 @@ namespace Dumux::Properties {
 ```
 ## Model type tag
 
-The property type tag is an empty struct with the respective name, e.g. `DiffusionModel`. 
+The property type tag is an empty struct with the respective name, e.g. `DiffusionModel`.
 All properties related to the desired model can be attributed using the property type tag.
 
 ```cpp
@@ -255,3 +255,5 @@ Implementation of a **nonlinear diffusion model** for denoising of an MRI image
 - Set model properties
 - Use model in test case
 - Customize volume variables
+
+* Go to [Model excercise](https://git.iws.uni-stuttgart.de/dumux-repositories/dumux-course/tree/master/exercises/exercise-model#exercise-model-dumux-course)
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