Returns a dictionary of the coordination numbers for each site label. e.g.::
        
            { 'A' : { 4 }, 'B' : { 2, 4 } }
 
        Args:
            none

        Returns:
            coordination_numbers (Dict(Str:Set(Int))): dictionary of coordination
                                                       numbers for each site label.

def site_coordination_numbers( self ):
        """
        Returns a dictionary of the coordination numbers for each site label. e.g.::
        
            { 'A' : { 4 }, 'B' : { 2, 4 } }
 
        Args:
            none

        Returns:
            coordination_numbers (Dict(Str:Set(Int))): dictionary of coordination
                                                       numbers for each site label.
        """
        coordination_numbers = {}
        for l in self.site_labels:
            coordination_numbers[ l ] = set( [ len( site.neighbours ) for site in self.sites if site.label is l ] ) 
        return coordination_numbers