small interface change to avoid covariant return types

dune/fem-dg/algorithm/base.hh

@@ -129,7 +129,7 @@ namespace Fem
     }
 
     //! return default data tuple for data output
-    //virtual IOTupleType dataTuple() = 0;
+    virtual IOTupleType dataTuple() = 0;
 
     // solve the problem for eoc loop 'loop'
     virtual void solve ( const int loop ) = 0;

dune/fem-dg/algorithm/steadystate.hh

@@ -161,11 +161,6 @@ namespace Fem
       return problem().dataPrefix();
     }
 
-    auto dataTuple () -> decltype( std::make_tuple( &this->solution() ))
-    {
-      return std::make_tuple( &solution_ );
-    }
-
     virtual SolverMonitorType& monitor()
     {
       return solverMonitorHandler_.monitor();