Assimilate the entire subdirectory structure in rootpath serially.

def serial_assimilate(self, rootpath):
        """
        Assimilate the entire subdirectory structure in rootpath serially.
        """
        valid_paths = []
        for (parent, subdirs, files) in os.walk(rootpath):
            valid_paths.extend(self._drone.get_valid_paths((parent, subdirs,
                                                            files)))
        data = []
        count = 0
        total = len(valid_paths)
        for path in valid_paths:
            newdata = self._drone.assimilate(path)
            self._data.append(newdata)
            count += 1
            logger.info('{}/{} ({:.2f}%) done'.format(count, total,
                                                      count / total * 100))
        for d in data:
            self._data.append(json.loads(d, cls=MontyDecoder))