Returns the nodes present in a particular timeslice

        Parameters
        ----------
        time_slice:int
                The timeslice should be a positive value greater than or equal to zero

        Examples
        --------
        >>> from pgmpy.models import DynamicBayesianNetwork as DBN
        >>> dbn = DBN()
        >>> dbn.add_nodes_from(['D', 'G', 'I', 'S', 'L'])
        >>> dbn.add_edges_from([(('D', 0),('G', 0)),(('I', 0),('G', 0)),(('G', 0),('L', 0)),(('D', 0),('D', 1))])
        >>> dbn.get_slice_nodes()

def get_slice_nodes(self, time_slice=0):
        """
        Returns the nodes present in a particular timeslice

        Parameters
        ----------
        time_slice:int
                The timeslice should be a positive value greater than or equal to zero

        Examples
        --------
        >>> from pgmpy.models import DynamicBayesianNetwork as DBN
        >>> dbn = DBN()
        >>> dbn.add_nodes_from(['D', 'G', 'I', 'S', 'L'])
        >>> dbn.add_edges_from([(('D', 0),('G', 0)),(('I', 0),('G', 0)),(('G', 0),('L', 0)),(('D', 0),('D', 1))])
        >>> dbn.get_slice_nodes()
        """
        if not isinstance(time_slice, int) or time_slice < 0:
            raise ValueError("The timeslice should be a positive value greater than or equal to zero")

        return [(node, time_slice) for node in self._nodes()]