Create download function for openiti markdown and tei output

The __download_part_output_transcription method is private cannot be called from the outside. Unless there's a security issue I'm not aware of, I propose making it public by removing the dunder "__" in front of the method name so that one can do

escr.download_part_output_transcription(document_pk,
                                        [part.pk for part in document_parts],
                                        transcription_layer.pk,
                                        output_type)

I also created a download_part_teixml_transcription() and download_part_openitimarkdown_transcription method in analogy to the download_part_pagexml_transcription etc. methods