[Fix] Sign of long wake in LongRangeResistiveWall

Fix wrong sign of the longitudinal wake in LongRangeResistiveWall, thanks to Keon Hee Kim for pointing this out.
Fix ImpedanceModel.add method to raise error before making modifications.

mbtrack2/impedance/impedance_model.py

@@ -102,8 +102,6 @@ class ImpedanceModel():
         None.
 
         """
-        self.wakefields.append(wakefield)
-        self.positions.append(positions)
         if name is None:
             name = wakefield.name
         if name is None:
@@ -112,6 +110,8 @@ class ImpedanceModel():
             self.names.append(name)
         else:
             raise ValueError("This name is already taken.")
+        self.wakefields.append(wakefield)
+        self.positions.append(positions)
 
     def add_global(self, wakefield, name=None):
         """

mbtrack2/tracking/wakepotential.py

@@ -793,7 +793,7 @@ class LongRangeResistiveWall(Element):
         for wake_type in self.types:
             kick = self.get_kick(rank, wake_type)
             if wake_type == "Wlong":
-                bunch["delta"] += kick / self.ring.E0
+                bunch["delta"] -= kick / self.ring.E0
             elif wake_type == "Wxdip":
                 bunch["xp"] += kick / self.ring.E0
             elif wake_type == "Wydip":