Retrieve pandas object stored in file

        Parameters
        ----------
        key : object

        Returns
        -------
        obj : same type as object stored in file

def get(self, key):
        """
        Retrieve pandas object stored in file

        Parameters
        ----------
        key : object

        Returns
        -------
        obj : same type as object stored in file
        """
        group = self.get_node(key)
        if group is None:
            raise KeyError('No object named {key} in the file'.format(key=key))
        return self._read_group(group)