Remove constructor for FV<K1,n> from FV<K2,m> with n!=m

This patch removes the constructor from the overload set.
Before the constructor was there but failed to compile
due to the static assertion.

If you have methods f(K) and f(FieldVector<K,n>) with
n>1 the overload was ambigous when calling f() with
a FieldVector<K,1> because there is a conversion
from this to FieldVector<K,n> - even if it does not
compile due to the failing static assertion.

Now this is avoided using SFINAE. This fixes fs#1024. 

[[Imported from SVN: r6680]]

dune/common/fvector.hh

@@ -10,6 +10,7 @@
 #include <complex>
 #include <cstring>
 
+#include "typetraits.hh"
 #include "exceptions.hh"
 #include "array.hh"
 #include "densevector.hh"
@@ -124,9 +125,19 @@ namespace Dune {
     FieldVector (const FieldVector & x) : _data(x._data)
     {}
 
-    //! Constructor making vector with identical coordinates
+    /**
+     * \brief Constructor making vector with identical coordinates
+     *
+     * If the DenseVector type of the this constructors argument
+     * is impemented by a FieldVector, it is statically checked
+     * if it has the correct size. If this is not the case
+     * the constructor is removed from the overload set using SFINAE.
+     *
+     * \param[in]  x  A DenseVector with correct size.
+     * \param[in]  dummy  A void* dummy argument needed by SFINAE.
+     */
     template<class C>
-    FieldVector (const DenseVector<C> & x)
+    FieldVector (const DenseVector<C> & x, typename Dune::enable_if<IsFieldVectorSizeCorrect<C,SIZE>::value>::type* dummy=0 )
     {
       dune_static_assert(((bool)IsFieldVectorSizeCorrect<C,SIZE>::value), "FieldVectors do not match in dimension!");
       assert(x.size() == SIZE);