docs: 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)