Implement addToDiagonal for scalars (interpreted as 1x1 matrices)

This is required for dune-istl matrices than use scalars to end
the nesting recursion (and not FieldMatrix), like BCRSMatrix<double>.

CHANGELOG.md

 # Master (will become release 2.8)
 
-- ...
+- The method `addToDiagonal` can now also be called if the matrix is a scalar number type.
+  This is needed since nowadays scalar entries can end the nesting recursion of dune-istl
+  matrices.
 
 ## Deprecations and removals
 

dune/matrix-vector/addtodiagonal.hh

@@ -11,6 +11,14 @@ namespace MatrixVector {
       x[i][i] += a;
   }
 
+  /** \brief Specialization for scalars (to be interpreted as 1x1 matrices) */
+  template <class Matrix>
+  static void addToDiagonal(Matrix& x, const Matrix& a)
+  {
+    static_assert(IsNumber<Matrix>::value, "Only scalars can be treated both as matrix and as number!");
+    x += a;
+  }
+
   /*
     the line
 

dune/matrix-vector/test/staticmatrixtoolstest.cc

@@ -35,6 +35,16 @@ private:
 
     // SQUARE MATRICES
 
+    // scalars
+    {
+      FieldType scalarMatrix_x = 1,
+                scalarMatrix_check = 4;
+
+      addToDiagonal(scalarMatrix_x, scalar_a);
+
+      passed = passed and
+               myDiff(scalarMatrix_x, scalarMatrix_check) < 1e-12;
+    }
     // case FM
     {
       Dune::FieldMatrix<FieldType, 2, 2> squareFieldMatrix_x = {{1, 2}, {3, 4}},