counts the number of documents in an URML file.
        adapted from Listing 2 on
        http://www.ibm.com/developerworks/library/x-hiperfparse/

def _get_num_of_documents(self):
        '''
        counts the number of documents in an URML file.
        adapted from Listing 2 on
        http://www.ibm.com/developerworks/library/x-hiperfparse/
        '''
        parser = etree.XMLParser(target=XMLElementCountTarget('document'))
        # When iterated over, 'results' will contain the output from
        # target parser's close() method
        num_of_documents = etree.parse(self.urml_file, parser)
        self._num_of_documents = num_of_documents
        return num_of_documents