def add_dr( self, dr ):
        """
        Add an observed interatomic distance to the g(r) data at dr.

        Args:
            dr (Float): the interatomic distance, dr.

        Returns:
            None
        """ 
        this_bin = int( dr / self.dr ) 
        if this_bin > self.number_of_bins:
            raise IndexError( 'dr is larger than rdf max_r' )
        self.data[ this_bin ] += 1
Add an observed interatomic distance to the g(r) data at dr.

        Args:
            dr (Float): the interatomic distance, dr.

        Returns:
            None