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)
@@ -301,14 +301,14 @@ class GiBUUOutput:
             
         """
         root_tupledata = self.arrays
-        energy_phase_space = self.flux_interpolation.integral(
-            self._energy_min, self._energy_max)
         xsec = self._event_xsec(
             root_tupledata
         ) * self.A  #xsec_per_nucleon * no_nucleons in the core
         if self.flux_data is not None:
             inv_gen_flux = np.power(
                 self.flux_interpolation(root_tupledata.lepIn_E), -1)
+            energy_phase_space = self.flux_interpolation.integral(
+                self._energy_min, self._energy_max)
             energy_factor = energy_phase_space * inv_gen_flux
         else:
             energy_factor = 1