Merge branch 'issue/missing_include' into 'develop'

Added missing include in PreconditionerInterface.hpp

See merge request spraetor/dune-amdis!33

src/amdis/linear_algebra/PreconditionerInterface.hpp

 #pragma once
 
+#include <amdis/Output.hpp>
+
 namespace AMDiS
 {
   /// Interface for Preconditioner types