load a MOC from a URL and display it in Aladin Lite widget
            Arguments:
            moc_URL: string url
            moc_options: dictionary object

def add_moc_from_URL(self, moc_URL, moc_options = {}):
        """ load a MOC from a URL and display it in Aladin Lite widget
            Arguments:
            moc_URL: string url
            moc_options: dictionary object"""
        self.moc_URL = moc_URL
        self.moc_options = moc_options
        self.moc_from_URL_flag = not self.moc_from_URL_flag