Add a child feature to this feature.

def add_child(self, child, rangecheck=False):
        """Add a child feature to this feature."""
        assert self.seqid == child.seqid, \
            (
                'seqid mismatch for feature {} ({} vs {})'.format(
                    self.fid, self.seqid, child.seqid
                )
            )
        if rangecheck is True:
            assert self._strand == child._strand, \
                ('child of feature {} has a different strand'.format(self.fid))
            assert self._range.contains(child._range), \
                (
                    'child of feature {} is not contained within its span '
                    '({}-{})'.format(self.fid, child.start, child.end)
                )
        if self.children is None:
            self.children = list()
        self.children.append(child)
        self.children.sort()