docs: def calc_signal_sum(self, measure='LFP'):
          """
          Superimpose each cell's contribution to the compound population signal,
          i.e., the population CSD or LFP
  
  
          Parameters
          ----------
          measure : str
              {'LFP', 'CSD'}: Either 'LFP' or 'CSD'.
  
  
          Returns
          -------
          numpy.ndarray
              The populations-specific compound signal.
  
          """
          #compute the total LFP of cells on this RANK
          if self.RANK_CELLINDICES.size > 0:
              for i, cellindex in enumerate(self.RANK_CELLINDICES):
                  if i == 0:
                      data = self.output[cellindex][measure]
                  else:
                      data += self.output[cellindex][measure]
          else:
              data = np.zeros((len(self.electrodeParams['x']),
                               self.cellParams['tstopms']/self.dt_output + 1),
                  dtype=np.float32)
  
          #container for full LFP on RANK 0
          if RANK == 0:
              DATA = np.zeros_like(data, dtype=np.float32)
          else:
              DATA = None
  
          #sum to RANK 0 using automatic type discovery with MPI
          COMM.Reduce(data, DATA, op=MPI.SUM, root=0)
  
          return DATA