Adds Variables to the probnet dict.

        Parameters
        ----------
        variable: <Element Variable at Variables Node in XML>
            etree Element consisting Variable tag.

        Examples
        -------
        >>> reader = ProbModelXMLReader()
        >>> reader.add_node(variable)

def add_node(self, variable):
        """
        Adds Variables to the probnet dict.

        Parameters
        ----------
        variable: <Element Variable at Variables Node in XML>
            etree Element consisting Variable tag.

        Examples
        -------
        >>> reader = ProbModelXMLReader()
        >>> reader.add_node(variable)
        """
        # TODO: Do some checks with variable type and roles. Right now I don't know when they are to be used.
        variable_name = variable.attrib['name']
        self.probnet['Variables'][variable_name] = {}
        self.probnet['Variables'][variable_name]['type'] = variable.attrib['type']
        self.probnet['Variables'][variable_name]['role'] = variable.attrib['role']
        if variable.find('Comment') is not None:
            self.probnet['Variables'][variable_name]['Comment'] = variable.find('Comment').text
        if variable.find('Coordinates') is not None:
            self.probnet['Variables'][variable_name]['Coordinates'] = variable.find('Coordinates').attrib
        if variable.find('AdditionalProperties/Property') is not None:
            self.probnet['Variables'][variable_name]['AdditionalProperties'] = {}
            for prop in variable.findall('AdditionalProperties/Property'):
                self.probnet['Variables'][variable_name]['AdditionalProperties'][prop.attrib['name']] = \
                    prop.attrib['value']
        if variable.find('States/State') is None:
            warnings.warn("States not available for node: " + variable_name)
        else:
            self.probnet['Variables'][variable_name]['States'] = {state.attrib['name']: {
                prop.attrib['name']: prop.attrib['value'] for
                prop in state.findall('AdditionalProperties/Property')} for state in variable.findall(
                'States/State')}