Fix bug in method RealTuple::log

Calling log(a,b) returned a-b instead of b-a.

Also, enable unit tests for the RealTuple class.  The current test
will not trigger the bug in the log method: that will come shortly.

Finally, this commit is the first that mentions its user-facing
changes in the newly introduced CHANGELOG.md file.

CHANGELOG.md

+# Master
+
+- Fix bug in the `RealTuple::log` method: Calling `log(a,b)`returned `a-b`
+  instead of `b-a`.
+

dune/gfe/realtuple.hh

@@ -110,10 +110,11 @@ public:
     }
 
     /** \brief The logarithmic map
-     * Simply the difference vector for RealTuple
+     *
+     * \result A vector in the tangent space of a, viz: b-a
      * */
     static auto log(const RealTuple& a, const RealTuple& b) {
-        return a.data_ - b.data_;
+        return b.data_ - a.data_;
     }
 
 #if ADOLC_ADOUBLE_H

test/targetspacetest.cc

@@ -355,8 +355,9 @@ void test()
 
 int main() try
 {
-//     test<RealTuple<double,1> >();
-//     test<RealTuple<double,3> >();
+    // Test the RealTuple class
+    test<RealTuple<double,1> >();
+    test<RealTuple<double,3> >();
     
     test<UnitVector<double,2> >();
     test<UnitVector<double,3> >();