[bugfix] make fvectortest compile and make ScalarOrderingTest pass for GMPField

the first problem was that I accidentially enabled the ScalarOrderingTest, but this forced me
to investigate further and I found out how this works, or not works.

We have to add the comparison operators to FieldVector, as the cast operator does not always
match, e.g. in the case of GMPField.
(cherry picked from commit e0ac8d9a)

Signed-off-by: Carsten Gräser <graeser@dune-project.org>

dune/common/fvector.hh

@@ -290,7 +290,35 @@ namespace Dune {
   };
 
   /* ----- FV / FV ----- */
-  /* not necessary as these operations are already covered via the cast operator */
+  /* mostly not necessary as these operations are already covered via the cast operator */
+
+  //! Binary compare, when using FieldVector<K,1> like K
+  template<class K>
+  inline bool operator> (const FieldVector<K,1>& a, const FieldVector<K,1>& b)
+  {
+    return a[0]>b[0];
+  }
+
+  //! Binary compare, when using FieldVector<K,1> like K
+  template<class K>
+  inline bool operator>= (const FieldVector<K,1>& a, const FieldVector<K,1>& b)
+  {
+    return a[0]>=b[0];
+  }
+
+  //! Binary compare, when using FieldVector<K,1> like K
+  template<class K>
+  inline bool operator< (const FieldVector<K,1>& a, const FieldVector<K,1>& b)
+  {
+    return a[0]<b[0];
+  }
+
+  //! Binary compare, when using FieldVector<K,1> like K
+  template<class K>
+  inline bool operator<= (const FieldVector<K,1>& a, const FieldVector<K,1>& b)
+  {
+    return a[0]<=b[0];
+  }
 
   /* ----- FV / scalar ----- */