Slides update

Mainly include file paths to shown code blocks. Also, fix minor display issues.

slides/materialsystem.md

@@ -63,10 +63,11 @@ static Scalar gasDensity(Scalar temperature, Scalar pressure)
 
 ```cpp
 static Scalar gasHeatCapacity(Scalar T, Scalar pressure)
-    {
-        const auto cp = shomateMethod.heatCapacity(T); // J/(mol K)
-        return cp / molarMass(); // J/(kg K)
-    }
+{
+    // apply Shomate method
+    const auto cp = shomateMethod.heatCapacity(T); // J/(mol K)
+    return cp / molarMass(); // J/(kg K)
+}
 
 ```
 <span style="font-size: 0.4em; position: relative; top: -38px; color: gray;">File: `dumux/dumux/material/components/ch4.hh`</span>