export DNA sequences from extensions and loopouts

[dave-doty]

This code crashes:

design = sc.Design(helices=[sc.Helix(max_offset=100)])
design.draw_strand(0, 0).extension_5p(3).move(5).with_sequence('TTT' + 'AAAAA')
contents = design.to_idt_bulk_input_format()

with this exception:

..\scadnano\scadnano.py:7369: in to_idt_bulk_input_format
    [strand.vendor_export_name(), strand.vendor_dna_sequence(domain_delimiter=domain_delimiter),
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = Strand(domains=[Extension(num_bases=3, display_length=1.0, display_angle=35.0, label=None, name=None, dna_sequence='TT...ields=None, is_scaffold=False, modification_5p=None, modification_3p=None, modifications_int={}, name=None, label=None)
domain_delimiter = ''

    def vendor_dna_sequence(self, domain_delimiter: str = '') -> str:
        """
        :param domain_delimiter:
            string to put in between DNA sequences of each domain, and between 5'/3' modifications and DNA.
            Note that the delimiter is not put between internal modifications and the next base(s)
            in the same domain.
        :return: DNA sequence as it needs to be typed to order from a DNA synthesis vendor, with
            :py:data:`Modification5Prime`'s,
            :py:data:`Modification3Prime`'s,
            and
            :py:data:`ModificationInternal`'s represented with text codes, e.g., for IDT DNA, using
            "/5Biosg/ACGT" for sequence ACGT with a 5' biotin modification.
        """
        self._ensure_modifications_legal(check_offsets_legal=True)

        if self.dna_sequence is None:
            raise ValueError('DNA sequence has not been assigned yet')

        ret_list: List[str] = []

        if self.modification_5p is not None and self.modification_5p.vendor_code is not None:
            ret_list.append(self.modification_5p.vendor_code)

        for substrand in self.domains:
>           ret_list.append(substrand.vendor_dna_sequence())
E           AttributeError: 'Extension' object has no attribute 'vendor_dna_sequence'

..\scadnano\scadnano.py:4362: AttributeError

This code also crashes:

design = sc.Design(helices=[sc.Helix(max_offset=100), sc.Helix(max_offset=100)])
design.draw_strand(0, 0).move(5).loopout(1, 3).move(-5).with_sequence('AAAAA' + 'TTT' + 'AAAAA')
contents = design.to_idt_bulk_input_format()

It's not happening in the web interface, yay strongly typed languages.