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

@@ -258,7 +258,7 @@ class GiBUUOutput:
 
     @property
     def mean_xsec(self):
-        if self.flux_data is None
+        if self.flux_data is None:
             return lambda energy: self.xsection["sum"]
         else
             root_tupledata = self.arrays