Output lat, lon coordinates as string in chosen format
        Inputs:
            format (str) - A string of the form A%B%C where A, B and C are identifiers.
              Unknown identifiers (e.g. ' ', ', ' or '_' will be inserted as separators
              in a position corresponding to the position in format.
        Examples:
            >> palmyra = LatLon(5.8833, -162.0833)
            >> palmyra.to_string('D') # Degree decimal output
            ('5.8833', '-162.0833')
            >> palmyra.to_string('H% %D')
            ('N 5.8833', 'W 162.0833')
            >> palmyra.to_string('d%_%M')
            ('5_52.998', '-162_4.998')

def to_string(self, format_str):
        '''
        Output lat, lon coordinates as string in chosen format
        Inputs:
            format (str) - A string of the form A%B%C where A, B and C are identifiers.
              Unknown identifiers (e.g. ' ', ', ' or '_' will be inserted as separators
              in a position corresponding to the position in format.
        Examples:
            >> palmyra = LatLon(5.8833, -162.0833)
            >> palmyra.to_string('D') # Degree decimal output
            ('5.8833', '-162.0833')
            >> palmyra.to_string('H% %D')
            ('N 5.8833', 'W 162.0833')
            >> palmyra.to_string('d%_%M')
            ('5_52.998', '-162_4.998')
        '''
        format2value = {'H': self.get_hemisphere(),
                        'M': abs(self.decimal_minute),
                        'm': int(abs(self.minute)),
                        'd': int(self.degree),
                        'D': self.decimal_degree,
                        'S': abs(self.second)}
        format_elements = format_str.split('%')
        coord_list = [str(format2value.get(element, element)) for element in format_elements]
        coord_str = ''.join(coord_list)
        if 'H' in format_elements: # No negative values when hemispheres are indicated
            coord_str = coord_str.replace('-', '')
        return coord_str