Returns the type of the root node of a DGParentedTree.

def get_node_type(dgtree):
    """Returns the type of the root node of a DGParentedTree."""
    if is_leaf(dgtree):
        return TreeNodeTypes.leaf_node

    root_label = dgtree.label()
    if root_label == '':
        assert dgtree == DGParentedTree('', []), \
            "The tree has no root label, but isn't empty: {}".format(dgtree)
        return TreeNodeTypes.empty_tree
    elif root_label in NUCLEARITY_LABELS:
        return TreeNodeTypes.nuclearity_node
    else:
        assert isinstance(dgtree, (RSTTree, DGParentedTree)), type(dgtree)
        return TreeNodeTypes.relation_node