[slides][model] Add file paths for code blocks.

slides/model.md

@@ -99,6 +99,8 @@ class DiffusionModelLocalResidual
     ...
 }
 ```
+<span style="font-size: 0.4em; position: relative; top: -38px; color: gray;">File: `dumux/examples/diffusion/model.hh`</span>
+
 Inherits from the `BaseLocalResidual`, which is chosen depending on the discretization scheme, here *Box scheme*.
 
 ## Storage term
@@ -113,6 +115,7 @@ NumEqVector computeStorage(const Problem& problem,
     return storage;
  }
 ```
+<span style="font-size: 0.4em; position: relative; top: -38px; color: gray;">File: `dumux/examples/diffusion/model.hh`</span>
 
 ## Flux term
 
@@ -142,6 +145,7 @@ NumEqVector computeFlux(const Problem& problem,
 }
 ```
 </font>
+<span style="font-size: 0.4em; position: relative; top: -60px; color: gray;">File: `dumux/examples/diffusion/model.hh`</span>
 
 ## Flux term
 ```cpp
@@ -161,6 +165,7 @@ NumEqVector computeFlux(...) const
     ...
 }
 ```
+<span style="font-size: 0.4em; position: relative; top: -38px; color: gray;">File: `dumux/examples/diffusion/model.hh`</span>
 
 ## Flux term
 ```cpp
@@ -179,6 +184,7 @@ NumEqVector computeFlux(...) const
     return flux;
 }
 ```
+<span style="font-size: 0.4em; position: relative; top: -38px; color: gray;">File: `dumux/examples/diffusion/model.hh`</span>
 
 ## `LocalResidual`
 
@@ -197,6 +203,8 @@ namespace Dumux::Properties {
     //define all properties
 } // end namespace Dumux::Properties
 ```
+<span style="font-size: 0.4em; position: relative; top: -38px; color: gray;">File: `dumux/examples/diffusion/model.hh`</span>
+
 ## Model type tag
 
 The property type tag is an empty struct with the respective name, e.g. `DiffusionModel`.
@@ -208,6 +216,7 @@ namespace Dumux::Properties::TTag {
 struct DiffusionModel {};
 } // end namespace Dumux::Properties::TTag
 ```
+<span style="font-size: 0.4em; position: relative; top: -38px; color: gray;">File: `dumux/examples/diffusion/model.hh`</span>
 
 ## Defining model properties
 
@@ -218,6 +227,7 @@ template<class TypeTag>
 struct LocalResidual<TypeTag, TTag::DiffusionModel>
 { using type = DiffusionModelLocalResidual<TypeTag>; };
 ```
+<span style="font-size: 0.4em; position: relative; top: -38px; color: gray;">File: `dumux/examples/diffusion/model.hh`</span>
 
 ## Defining model properties
 
@@ -238,6 +248,8 @@ struct ModelTraits<TypeTag, TTag::DiffusionModel>
     };
 };
 ```
+<span style="font-size: 0.4em; position: relative; top: -38px; color: gray;">File: `dumux/examples/diffusion/model.hh`</span>
+
 ## Defining model properties
 Further model specific properties can be set accordingly by using the model property tag,
 i.e. `TTag::DiffusionModel`