Return a pytables column object.

        :Parameters:
        chain : integer
          The index of the chain.

        .. note::
           This method is specific to the ``hdf5`` backend.

def hdf5_col(self, chain=-1):
        """Return a pytables column object.

        :Parameters:
        chain : integer
          The index of the chain.

        .. note::
           This method is specific to the ``hdf5`` backend.
        """
        return self.db._tables[chain].colinstances[self.name]