[fieldvector] add constructors for scalar fieldvector, matching all compatible scalar data types

in order to make GMPField work in more situations, we add a constrctor, which matches
all scalar data types, which can me converted to K.
We deliberately exclude K from the template match.

dune/common/fvector.hh

@@ -228,7 +228,14 @@ namespace Dune {
     {}
 
     /** \brief Constructor with a given scalar */
-    FieldVector (const K& k) : _data(k) {}
+    template<typename T,
+             typename EnableIf = typename std::enable_if<
+               std::is_convertible<T, K>::value &&
+               ! std::is_same<K, DenseVector<typename FieldTraits<T>::field_type>
+                              >::value
+               >::type
+             >
+    FieldVector (const T& k) : _data(k) {}
 
     //! Constructor making vector with identical coordinates
     template<class C>
@@ -245,7 +252,14 @@ namespace Dune {
     {}
 
     //! Assignment operator for scalar
-    inline FieldVector& operator= (const K& k)
+    template<typename T,
+             typename EnableIf = typename std::enable_if<
+               std::is_convertible<T, K>::value &&
+               ! std::is_same<K, DenseVector<typename FieldTraits<T>::field_type>
+                              >::value
+               >::type
+             >
+    inline FieldVector& operator= (const T& k)
     {
       _data = k;
       return *this;