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

@@ -259,7 +259,7 @@ class GiBUUOutput:
     @property
     def mean_xsec(self):
         if self.flux_data is None:
-            return self._event_xsec(self.arrays)
+            return lambda energy: self.energy_max
         root_tupledata = self.arrays
         energies = np.array(root_tupledata.lepIn_E)
         weights = self._event_xsec(root_tupledata)