docs: def _syscal_write_electrode_coords(fid, spacing, N):
      """helper function that writes out electrode positions to a file descriptor
  
      Parameters
      ----------
      fid: file descriptor
          data is written here
      spacing: float
          spacing of electrodes
      N: int
          number of electrodes
      """
      fid.write('# X Y Z\n')
      for i in range(0, N):
          fid.write('{0} {1} {2} {3}\n'.format(i + 1, i * spacing, 0, 0))