Updates `self.pathways` and `self.n_pathways.`

        Parameters
        -----------
        pathway : str
          the pathway to add to the network.

def add_pathway(self, pathway):
        """Updates `self.pathways` and `self.n_pathways.`

        Parameters
        -----------
        pathway : str
          the pathway to add to the network.
        """
        if pathway not in self.pathways:
            self.pathways[pathway] = self.n_pathways
            self.n_pathways += 1
        return self.pathways[pathway]