Add variables to XMLBIF

        Return
        ------
        dict: dict of type {variable: variable tags}

        Examples
        --------
        >>> writer = XMLBIFWriter(model)
        >>> writer.get_variables()
        {'bowel-problem': <Element VARIABLE at 0x7fe28607dd88>,
         'family-out': <Element VARIABLE at 0x7fe28607de08>,
         'hear-bark': <Element VARIABLE at 0x7fe28607de48>,
         'dog-out': <Element VARIABLE at 0x7fe28607ddc8>,
         'light-on': <Element VARIABLE at 0x7fe28607de88>}

def get_variables(self):
        """
        Add variables to XMLBIF

        Return
        ------
        dict: dict of type {variable: variable tags}

        Examples
        --------
        >>> writer = XMLBIFWriter(model)
        >>> writer.get_variables()
        {'bowel-problem': <Element VARIABLE at 0x7fe28607dd88>,
         'family-out': <Element VARIABLE at 0x7fe28607de08>,
         'hear-bark': <Element VARIABLE at 0x7fe28607de48>,
         'dog-out': <Element VARIABLE at 0x7fe28607ddc8>,
         'light-on': <Element VARIABLE at 0x7fe28607de88>}
        """
        variables = self.model.nodes()
        variable_tag = {}
        for var in sorted(variables):
            variable_tag[var] = etree.SubElement(self.network, "VARIABLE", attrib={'TYPE': 'nature'})
            etree.SubElement(variable_tag[var], "NAME").text = var
        return variable_tag