Calculates the frequency of salt bridges throughout simulations. If the frequency exceeds the
        analysis cutoff, this interaction will be taken for further consideration.
        Takes:
            * analysis_cutoff * - fraction of simulation time a feature has to be present for to be plotted
        Output:
            * self.saltbridge_frequency * - frequency of each salt bridge

def get_saltbridge_frequency(self,analysis_cutoff):
        """Calculates the frequency of salt bridges throughout simulations. If the frequency exceeds the
        analysis cutoff, this interaction will be taken for further consideration.
        Takes:
            * analysis_cutoff * - fraction of simulation time a feature has to be present for to be plotted
        Output:
            * self.saltbridge_frequency * - frequency of each salt bridge """
        self.frequency = defaultdict(int)
        for traj in self.saltbridges_by_type:
            for contact in self.saltbridges_by_type[traj]:
                self.frequency[contact["ligand_atom_id"],contact["ligand_atom_name"],contact["resid"],contact["resname"],contact["segid"]]+=contact["frequency"]
        draw_frequency = {i:self.frequency[i] for i in self.frequency if self.frequency[i]>(int(len(self.trajectory))*analysis_cutoff)}

        self.saltbridges_for_drawing = {}
        for contact in draw_frequency:
            self.saltbridges_for_drawing[contact]=draw_frequency[contact]