Implement two-argument vector / scalar multiplication

This is implemented for FieldVector only: For general
DenseVectors it is currently not possible to implement
the necessary type promotion.

CHANGELOG.md

@@ -29,6 +29,11 @@
   now have additional operators.  In particular, there are
   - Vector = - Vector
   - Matrix = - Matrix
+  While these two work for any vector or matrix class that inherits from `DenseVector`
+  or `DenseMatrix`, the following additional methods only work for `FieldVector`:
+  - Vector = Scalar * Vector
+  - Vector = Vector * Scalar
+  - Vector = Vector / Scalar
 
 - There is now (finally!) a method `power` in the file `math.hh` that computes
   powers with an integer exponent, and is usable in compile-time expressions.

dune/common/fvector.hh

@@ -22,6 +22,7 @@
 #include "boundschecking.hh"
 
 #include <dune/common/math.hh>
+#include <dune/common/promotiontraits.hh>
 
 namespace Dune {
 
@@ -208,6 +209,46 @@ namespace Dune {
     {
       return _data.data();
     }
+
+    //! vector space multiplication with scalar
+    template <class Scalar,
+              std::enable_if_t<IsNumber<Scalar>::value, int> = 0>
+    friend auto operator* ( const FieldVector& vector, Scalar scalar)
+    {
+      FieldVector<typename PromotionTraits<value_type,Scalar>::PromotedType,SIZE> result;
+
+      for (size_type i = 0; i < vector.size(); ++i)
+        result[i] = vector[i] * scalar;
+
+      return result;
+    }
+
+    //! vector space multiplication with scalar
+    template <class Scalar,
+              std::enable_if_t<IsNumber<Scalar>::value, int> = 0>
+    friend auto operator* ( Scalar scalar, const FieldVector& vector)
+    {
+      FieldVector<typename PromotionTraits<value_type,Scalar>::PromotedType,SIZE> result;
+
+      for (size_type i = 0; i < vector.size(); ++i)
+        result[i] = scalar * vector[i];
+
+      return result;
+    }
+
+    //! vector space division by scalar
+    template <class Scalar,
+              std::enable_if_t<IsNumber<Scalar>::value, int> = 0>
+    friend auto operator/ ( const FieldVector& vector, Scalar scalar)
+    {
+      FieldVector<typename PromotionTraits<value_type,Scalar>::PromotedType,SIZE> result;
+
+      for (size_type i = 0; i < vector.size(); ++i)
+        result[i] = vector[i] * scalar;
+
+      return result;
+    }
+
   };
 
   /** \brief Read a FieldVector from an input stream

dune/common/test/fvectortest.cc

@@ -101,6 +101,9 @@ struct FieldVectorMainTestCommons
     w -= a;
     w *= a;
     w /= a;
+    w = a * v;
+    w = v * a;
+    w = v / a;
 
     // Negation
     -v;