Fix scaling of the FD gradient used for testing

The formula for the FD gradient never took the distance between
the two Lagrange points into account, even though the gradient
of an interpolation function clearly depends on it.

(cherry picked from commit 7f326ded)

test/rotationtest.cc

@@ -181,6 +181,10 @@ void testDerivativeOfInterpolatedPosition()
                         
                     }
                     
+                    // Scale the finite difference gradient with the interval length
+                    for (auto& parDer : fdGrad)
+                        parDer /= intervalLength;
+
                     // Compute analytical velocity vector gradient
                     RodLocalStiffness<OneDGrid,double>::interpolationVelocityDerivative(q[i], q[j], s, intervalLength, grad);