Change sign of LongitudinalMap dtau/dt equation

Compared to mbtrack, the head/tail convention is reversed which leads to this sign change.
In this python code, the head is for negative times, t<0.
The sign change allows successful tracking benchmark with mbtrack for pure inductive and resonator impedance

tracking/element.py

@@ -93,7 +93,7 @@ class LongitudinalMap(Element):
         bunch : Bunch or Beam object
         """
         bunch["delta"] -= self.ring.U0 / self.ring.E0
-        bunch["tau"] -= self.ring.ac * self.ring.T0 * bunch["delta"]
+        bunch["tau"] += self.ring.ac * self.ring.T0 * bunch["delta"]
 
 class SynchrotronRadiation(Element):
     """