Add docstring to gun.momentumMin; default is 0.0

DDG4/python/DDSim/Helper/Gun.py

@@ -26,6 +26,7 @@ class Gun(ConfigHelper):
     self.phiMax = None
     self.thetaMin = None
     self.thetaMax = None
+    self._momentumMin_EXTRA = {'help': "Minimal momentum when using distribution (default = 0.0)"}
     self.momentumMin = None
     self.momentumMax = None
 
@@ -113,8 +114,8 @@ class Gun(ConfigHelper):
   def setOptions(self, ddg4Gun):
     """set the starting properties of the DDG4 particle gun"""
     try:
-      ddg4Gun.energy = self.energy # ddg4Gun.energy actually sets momentum
-      ddg4Gun.MomentumMin = self.energy
+      ddg4Gun.energy = self.energy  # ddg4Gun.energy actually sets momentum
+      ddg4Gun.MomentumMin = 0.0
       ddg4Gun.MomentumMax = self.energy
       ddg4Gun.particle = self.particle
       ddg4Gun.multiplicity = self.multiplicity