Draft: Remove FieldMatrix/Vector scalar specialization

(This is an experiment whether it would be possible to remove the FieldVector<K,1> and FieldMatrix<K,1,1> specialization completely.)

Requires:

• dune-common!1499

dune/grid/io/file/vtk/function.hh

@@ -246,7 +246,7 @@ namespace Dune
 
       // (Ab)use the MultiLinearGeometry class to do multi-linear interpolation between scalars
       const MultiLinearGeometry<ctype,dim,1> interpolation(e.type(), cornerValues);
-      return interpolation.global(xi);
+      return interpolation.global(xi)[0];
     }
 
     //! get name