'Write accumulated ncx_soup to file.
Expanded description
Inputs:
catalog_path (str): path to generated catalog
opts.basename (str): catalog basename
Output:
(file): basename.NCX written'

def write_ncx(self):

    self.update_progress_full_step(_('Saving    NCX'))
    outfile = open(('%s/%s.ncx' % (self.catalog_path, self.opts.basename)), 'w')
    outfile.write(self.ncx_soup.prettify())
