Feature/cleanup of eigenvalue solver docs

improve structure of the doxygen documentation to make the eigenvalue solvers visible and hide implementation details

dune/istl/eigenvalue/poweriteration.hh

@@ -997,7 +997,7 @@ namespace Dune
           entry = m_entry - mu_;
         }
         // notify linear solver about change of the iteration matrix object
-        Impl::SolverHelper<ISTLLinearSolver,BCRSMatrix>::setMatrix
+        SolverHelper<ISTLLinearSolver,BCRSMatrix>::setMatrix
           (solver,*itMatrix_);
       }
     }