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 }

def site_occupation_statistics( self ):
        """
        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 } 
        """
        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