Add a new item (a catalogue or resource) as a child of this catalogue.

def addItem(self, child, href):
        """Add a new item (a catalogue or resource) as a child of this catalogue."""
        assert isinstance(child, Base), "child must be a hypercat Catalogue or Resource"
        child.setHref(href)
        for item in self.items:
            assert item.href != href, "All items in a catalogue must have unique hrefs : "+href
        self.items += [child]           # Add new
        return