Create detector from detx file.

def _init_from_file(self, filename):
        """Create detector from detx file."""
        if not filename.endswith("detx"):
            raise NotImplementedError('Only the detx format is supported.')
        self._open_file(filename)
        self._extract_comments()
        self._parse_header()
        self._parse_doms()
        self._det_file.close()