for fmatrix<1,1> the scalar constructor doesn't need to be explicit

[[Imported from SVN: r6184]]

dune/common/fmatrix.hh

@@ -219,7 +219,7 @@ namespace Dune
       _data[0] = k;
     }
     template<typename T>
-    explicit FieldMatrix(const T& t)
+    FieldMatrix(const T& t)
     {
       DenseMatrixAssigner<Conversion<T,K>::exists>::assign(*this, t);
     }