Fix method 'difference'

When switching to the double covered quaternion pair, the corresponding distance was not computed correctly

test/valuefactory.hh

@@ -158,8 +158,8 @@ public:
         std::vector<std::array<double,4> > testPoints = {{1,0,0,0}, {0,1,0,0}, {-0.838114,0.356751,-0.412667,0.5},
                                     {-0.490946,-0.306456,0.81551,0.23},{-0.944506,0.123687,-0.304319,-0.7},
                                     {-0.6,0.1,-0.2,0.8},{0.45,0.12,0.517,0},
-                                    {-0.1,0.3,-0.1,0.73},{-0.444506,0.123687,0.104319,-0.23},{-0.7,-0.123687,-0.304319,0.72}};
-
+                                    {-0.1,0.3,-0.1,0.73},{-0.444506,0.123687,0.104319,-0.23},{-0.7,-0.123687,-0.304319,0.72},
+                                    {-0.035669, -0.463824, -0.333265, 0.820079}, {0.0178678, 0.916836, 0.367358, 0.155374}};
 
         values.resize(testPoints.size());