[Cleanup] Header guards

src/friction_writer.hh

-//#include <iostream>
+#ifndef FRICTION_WRITER_HH
+#define FRICTION_WRITER_HH
+
 #include <fstream>
 
 #include <dune/tectonic/frictiondata.hh>
@@ -21,3 +23,4 @@ template <class BitVectorType> class FrictionWriter {
   FrictionData const &fd;
   BitVectorType const &boundaryNodes;
 };
+#endif