docs: def gen_schlumberger(self, M, N, a=None):
          """generate one Schlumberger sounding configuration, that is, one set
          of configurations for one potential dipole MN.
  
          Parameters
          ----------
          M: int
              electrode number for the first potential electrode
          N: int
              electrode number for the second potential electrode
          a: int, optional
              stepping between subsequent voltage electrodes. If not set,
              determine it as a = abs(M - N)
  
          Returns
          -------
          configs: Kx4 numpy.ndarray
              array holding the configurations
  
          Examples
          --------
  
              import reda.configs.configManager as CRconfig
              config = CRconfig.ConfigManager(nr_of_electrodes=40)
              config.gen_schlumberger(M=20, N=21)
  
          """
          if a is None:
              a = np.abs(M - N)
  
          nr_of_steps_left = int(min(M, N) - 1 / a)
          nr_of_steps_right = int((self.nr_electrodes - max(M, N)) / a)
          configs = []
          for i in range(0, min(nr_of_steps_left, nr_of_steps_right)):
              A = min(M, N) - (i + 1) * a
              B = max(M, N) + (i + 1) * a
              configs.append((A, B, M, N))
          configs = np.array(configs)
          self.add_to_configs(configs)
          return configs