Count how many salt bridges occured in each frame.
        Returns numpy array.

def count_by_time(self):
        """Count how many salt bridges occured in each frame.
        Returns numpy array."""
        out = np.empty((len(self.timesteps),), dtype=[('time', float), ('count', int)])
        for cursor,timestep in enumerate(self.timesteps):
            out[cursor] = (timestep,len([x for x in self.timeseries if x.time==timestep]))
        return out.view(np.recarray)