Create a dictionary with the EVT header information

def extract_header(self):
        """Create a dictionary with the EVT header information"""
        self.log.info("Extracting the header")
        raw_header = self.raw_header = defaultdict(list)
        first_line = self.blob_file.readline()
        first_line = try_decode_string(first_line)
        self.blob_file.seek(0, 0)
        if not first_line.startswith(str('start_run')):
            self.log.warning("No header found.")
            return raw_header
        for line in iter(self.blob_file.readline, ''):
            line = try_decode_string(line)
            line = line.strip()
            try:
                tag, value = str(line).split(':')
            except ValueError:
                continue
            raw_header[tag].append(str(value).split())
            if line.startswith(str('end_event:')):
                self._record_offset()
                if self._auto_parse and 'physics' in raw_header:
                    parsers = [p[0].lower() for p in raw_header['physics']]
                    self._register_parsers(parsers)
                return raw_header
        raise ValueError("Incomplete header, no 'end_event' tag found!")