implement leapfrog and stormer_verlet

jobs/tests/test_integrator.py

@@ -297,5 +297,8 @@ class IntegratorTest(unittest.TestCase):
                                      t=np.linspace(0, T, int(T // dt)),
                                      force=force,
                                      solver=stormer_verlet)
+        t, p_reverse, q_reverse = system.direct_simulation()
+        self.assertTrue(np.greater(1e-10 + np.zeros(4), q_reverse[-1] - q[0]).all())
+        self.assertTrue(np.greater(1e-10 + np.zeros(4), p_reverse[-1] - p[0]).all())
 if __name__ == '__main__':
     unittest.main()