Returns the categories of `Ligands` in `LigandGroup`.

def categories(self):
        """Returns the categories of `Ligands` in `LigandGroup`."""
        category_dict = {}
        for ligand in self:
            if ligand.category in category_dict:
                category_dict[ligand.category].append(ligand)
            else:
                category_dict[ligand.category] = [ligand]
        return category_dict