Revert last patch---it was a bad idea.

Even though multiplication of Rotations is the same thing
as multiplication of Quaternions, in the first case the
output is a Rotation class, and in the second on a Quaternion
class.

[[Imported from SVN: r8343]]

dune/gfe/rotation.hh

@@ -208,6 +208,17 @@ public:
         return id;
     }
 
+    /** \brief Right multiplication */
+    Rotation<T,3> mult(const Rotation<T,3>& other) const {
+        Rotation<T,3> q;
+        q[0] =   (*this)[3]*other[0] - (*this)[2]*other[1] + (*this)[1]*other[2] + (*this)[0]*other[3];
+        q[1] =   (*this)[2]*other[0] + (*this)[3]*other[1] - (*this)[0]*other[2] + (*this)[1]*other[3];
+        q[2] = - (*this)[1]*other[0] + (*this)[0]*other[1] + (*this)[3]*other[2] + (*this)[2]*other[3];
+        q[3] = - (*this)[0]*other[0] - (*this)[1]*other[1] - (*this)[2]*other[2] + (*this)[3]*other[3];
+
+        return q;
+    }
+
     /** \brief The exponential map from \f$ \mathfrak{so}(3) \f$ to \f$ SO(3) \f$
      */
     static Rotation<T,3> exp(const SkewMatrix<T,3>& v) {