docs: 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