code: def initialise_site_lookup_table( self ):
          
          self.site_lookup = {}
          for site in self.sites:
              self.site_lookup[ site.number ] = site
docs: Create a lookup table allowing sites in this lattice to be queried using `self.site_lookup[n]` where `n` is the identifying site numbe.
  
          Args:
              None
  
          Returns:
              None