code: def site_occupation_statistics( self ):
          
          if self.time == 0.0:
              return None
          occupation_stats = { label : 0.0 for label in self.site_labels }
          for site in self.sites:
              occupation_stats[ site.label ] += site.time_occupied
          for label in self.site_labels:
              occupation_stats[ label ] /= self.time
          return occupation_stats
docs: Average site occupation for each site type
  
          Args:
              None
  
          Returns:
              (Dict(Str:Float)): Dictionary of occupation statistics, e.g.::
  
                  { 'A' : 2.5, 'B' : 25.3 }