Single energy runs

GiBUU provides the possibility to run at single energy, which provides different output files. This should be covered with KM3BUU.

km3buu/output.py

@@ -260,7 +260,7 @@ class GiBUUOutput:
     def mean_xsec(self):
         if self.flux_data is None:
             return lambda energy: self.xsection["sum"]
-        else
+        else:
             root_tupledata = self.arrays
             energies = np.array(root_tupledata.lepIn_E)
             weights = self._event_xsec(root_tupledata)