load a VOTable table from an url and load its data into the widget 
            Args:
                votable_URL: string url
                votable_options: dictionary object

def add_catalog_from_URL(self, votable_URL, votable_options={}):
        """ load a VOTable table from an url and load its data into the widget 
            Args:
                votable_URL: string url
                votable_options: dictionary object"""
        self.votable_URL= votable_URL
        self.votable_options= votable_options
        self.votable_from_URL_flag= not self.votable_from_URL_flag