How to use the nexusformat.nexus.NXgroup function in nexusformat

def sync_children(self, item):
        if isinstance(item.node, NXgroup):
            children = []
            if item.hasChildren():
                for row in range(item.rowCount()):
                    children.append(item.child(row))
            names = [child.text() for child in children]
            for name in item.node:
                if name not in names:
                    item.appendRow(NXTreeItem(item.node[name]))
            for child in children:
                name = child.node.nxname
                if name not in item.node:
                    item.removeRow(child.row())
                elif child.node is not item.node[name]:
                    item.removeRow(child.row())
                    item.appendRow(NXTreeItem(item.node[name]))
        item.node.set_unchanged()

def status_message(self, message):
        if isinstance(message, NXfield) or isinstance(message, NXgroup):
            text = message._str_name()+' '+message._str_attrs()
        elif isinstance(message, NXlink):
            text = message._str_name()
        else:
            text = str(message)
        self.mainwindow.statusBar().showMessage(text.replace('\n','; '))

from __future__ import (absolute_import, division, print_function,
                        unicode_literals)

import os

import pkg_resources
import six
from nexusformat.nexus import (NeXusError, NXdata, NXentry, NXfield, NXgroup,
                               NXlink, NXroot, nxload)

from .pyqt import QtCore, QtGui, QtWidgets
from .utils import display_message, natural_sort, modification_time
from nexusformat.nexus import *


class NXtree(NXgroup):
    """
    NXtree group. This is a subclass of the NXgroup class.

    It is used as the invisible root item for NeXpy tree views.
    """
    _model = None
    _view = None
    _item = None
    _shell = {}
    _attrs = {}

    def __init__(self):
        self._class = 'NXtree'
        self._name = 'tree'
        self._entries = {}

def accept(self):
        name = self.get_name()
        if self.class_name == "NXgroup":
            nxclass = self.get_type()
            if name:
                self.node[name] = NXgroup(nxclass=nxclass)
            else:
                self.node.insert(NXgroup(nxclass=nxclass))
            logging.info("'%s' added to '%s'" 
                         % (self.node[name], self.node.nxpath)) 
        elif name:
            value = self.get_value()
            dtype = self.get_type()
            if value is not None:
                if self.class_name == "NXfield":
                    self.node[name] = NXfield(value, dtype=dtype)
                    logging.info("'%s' added to '%s'" 
                                 % (name, self.node.nxpath)) 
                    units = self.get_units()
                    if units:
                        self.node[name].attrs['units'] = units
                else:
                    self.node.attrs[name] = NXattr(value, dtype=dtype)
                    logging.info("Attribute '%s' added to '%s'"

def check_axis(self, node, axis):
        if isinstance(node, NXgroup) or node.ndim > 1:
            return False
        axis_len = self.signal.shape[axis]
        if node.ndim == 0:
            node_len = 1
        else:
            node_len = node.shape[0]
        if node_len == axis_len or node_len == axis_len+1:
            return True
        else:
            return False

def accept(self):
        name = self.get_name()
        if self.class_name == "NXgroup":
            nxclass = self.get_type()
            if name:
                self.node[name] = NXgroup(nxclass=nxclass)
            else:
                self.node.insert(NXgroup(nxclass=nxclass))
            logging.info("'%s' added to '%s'" 
                         % (self.node[name], self.node.nxpath)) 
        elif name:
            value = self.get_value()
            dtype = self.get_type()
            if value is not None:
                if self.class_name == "NXfield":
                    self.node[name] = NXfield(value, dtype=dtype)
                    logging.info("'%s' added to '%s'" 
                                 % (name, self.node.nxpath)) 
                    units = self.get_units()
                    if units:
                        self.node[name].attrs['units'] = units
                else: