refactor: Add method to reset jacobian in stepper (#1171)

When trying to use the ATLAS stepper with the KF I noticed that it wouldn't work since we use to set the stepping state jacobian to a BoundMatrix::Identity() which isn't compatible with the ATLAS stepper.
This PR add a method to the stepper to set the jacobian to the identity, and thus solve this issue. 
Similarly I had to transform the update to a reset for the propagation to the Perigee as it relied on manually setting some matrix afterward (which was not compatible with the ATLAS stepper).

Core/include/Acts/Propagator/AtlasStepper.hpp

@@ -1376,6 +1376,54 @@ class AtlasStepper {
     return h;
   }
 
+  /// Method that reset the Jacobian to the Identity for when no bound state are
+  /// available
+  ///
+  /// @param [in,out] state State of the stepper
+  void setIdentityJacobian(State& state) const {
+    state.jacobian[0] = 1.;  // dL0/dL0
+    state.jacobian[1] = 0.;  // dL0/dL1
+    state.jacobian[2] = 0.;  // dL0/dPhi
+    state.jacobian[3] = 0.;  // dL0/dThe
+    state.jacobian[4] = 0.;  // dL0/dCM
+    state.jacobian[5] = 0.;  // dL0/dT
+
+    state.jacobian[6] = 0.;   // dL1/dL0
+    state.jacobian[7] = 1.;   // dL1/dL1
+    state.jacobian[8] = 0.;   // dL1/dPhi
+    state.jacobian[9] = 0.;   // dL1/dThe
+    state.jacobian[10] = 0.;  // dL1/dCM
+    state.jacobian[11] = 0.;  // dL1/dT
+
+    state.jacobian[12] = 0.;  // dPhi/dL0
+    state.jacobian[13] = 0.;  // dPhi/dL1
+    state.jacobian[14] = 1.;  // dPhi/dPhi
+    state.jacobian[15] = 0.;  // dPhi/dThe
+    state.jacobian[16] = 0.;  // dPhi/dCM
+    state.jacobian[17] = 0.;  // dPhi/dT
+
+    state.jacobian[18] = 0.;  // dThe/dL0
+    state.jacobian[19] = 0.;  // dThe/dL1
+    state.jacobian[20] = 0.;  // dThe/dPhi
+    state.jacobian[21] = 1.;  // dThe/dThe
+    state.jacobian[22] = 0.;  // dThe/dCM
+    state.jacobian[23] = 0.;  // dThe/dT
+
+    state.jacobian[24] = 0.;  // dCM /dL0
+    state.jacobian[25] = 0.;  // dCM /dL1
+    state.jacobian[26] = 0.;  // dCM /dPhi
+    state.jacobian[27] = 0.;  // dCM /dTheta
+    state.jacobian[28] = 1.;  // dCM /dCM
+    state.jacobian[29] = 0.;  // dCM/dT
+
+    state.jacobian[30] = 0.;  // dT/dL0
+    state.jacobian[31] = 0.;  // dT/dL1
+    state.jacobian[32] = 0.;  // dT/dPhi
+    state.jacobian[33] = 0.;  // dT/dThe
+    state.jacobian[34] = 0.;  // dT/dCM
+    state.jacobian[35] = 1.;  // dT/dT
+  }
+
  private:
   std::shared_ptr<const MagneticFieldProvider> m_bField;
 

Core/include/Acts/Propagator/EigenStepper.hpp

@@ -387,6 +387,12 @@ class EigenStepper {
   template <typename propagator_state_t>
   Result<double> step(propagator_state_t& state) const;
 
+  /// Method that reset the Jacobian to the Identity for when no bound state are
+  /// available
+  ///
+  /// @param [in,out] state State of the stepper
+  void setIdentityJacobian(State& state) const;
+
  protected:
   /// Magnetic field inside of the detector
   std::shared_ptr<const MagneticFieldProvider> m_bField;

Core/include/Acts/Propagator/EigenStepper.ipp

@@ -265,3 +265,8 @@ Acts::Result<double> Acts::EigenStepper<E, A>::step(
   }
   return h;
 }
+
+template <typename E, typename A>
+void Acts::EigenStepper<E, A>::setIdentityJacobian(State& state) const {
+  state.jacobian = BoundMatrix::Identity();
+}

Core/include/Acts/TrackFitting/KalmanFitter.hpp

@@ -814,7 +814,7 @@ class KalmanFitter {
         }
         // Not creating bound state here, so need manually reinitialize
         // jacobian
-        state.stepping.jacobian = BoundMatrix::Identity();
+        stepper.setIdentityJacobian(state.stepping);
         if (surface->surfaceMaterial() != nullptr) {
           // Update state and stepper with material effects
           materialInteractor(surface, state, stepper);
@@ -972,16 +972,14 @@ class KalmanFitter {
           (std::abs(firstIntersection.intersection.pathLength) <=
            std::abs(lastIntersection.intersection.pathLength));
       if (closerTofirstCreatedState) {
-        stepper.update(state.stepping, firstParams,
-                       firstCreatedState.smoothed(),
-                       firstCreatedState.smoothedCovariance(),
-                       firstCreatedState.referenceSurface());
+        stepper.resetState(state.stepping, firstCreatedState.smoothed(),
+                           firstCreatedState.smoothedCovariance(),
+                           firstCreatedState.referenceSurface());
         reverseDirection = (firstIntersection.intersection.pathLength < 0);
       } else {
-        stepper.update(state.stepping, lastParams,
-                       lastCreatedMeasurement.smoothed(),
-                       lastCreatedMeasurement.smoothedCovariance(),
-                       lastCreatedMeasurement.referenceSurface());
+        stepper.resetState(state.stepping, lastCreatedMeasurement.smoothed(),
+                           lastCreatedMeasurement.smoothedCovariance(),
+                           lastCreatedMeasurement.referenceSurface());
         reverseDirection = (lastIntersection.intersection.pathLength < 0);
       }
       const auto& surface = closerTofirstCreatedState
@@ -1000,10 +998,6 @@ class KalmanFitter {
         state.stepping.navDir =
             (state.stepping.navDir == forward) ? backward : forward;
       }
-      // Reinitialize the stepping jacobian
-      state.stepping.jacobian = BoundMatrix::Identity();
-      state.stepping.jacTransport = FreeMatrix::Identity();
-      state.stepping.derivative = FreeVector::Zero();
       // Reset the step size
       state.stepping.stepSize = ConstrainedStep(
           state.stepping.navDir * std::abs(state.options.maxStepSize));