docs: def read_voltages(self, voltage_file):
          """import voltages from a volt.dat file
  
          Parameters
          ----------
          voltage_file : string
              Path to volt.dat file
          """
  
          measurements_raw = np.loadtxt(
              voltage_file,
              skiprows=1,
          )
          measurements = np.atleast_2d(measurements_raw)
  
          # extract measurement configurations
          A = (measurements[:, 0] / 1e4).astype(int)
          B = (measurements[:, 0] % 1e4).astype(int)
          M = (measurements[:, 1] / 1e4).astype(int)
          N = (measurements[:, 1] % 1e4).astype(int)
          ABMN = np.vstack((A, B, M, N)).T
  
          if self.configs.configs is None:
              self.configs.configs = ABMN
          else:
              # configurations don't match
              if not np.all(ABMN == self.configs.configs):
                  for nr, (old_config, new_config) in enumerate(zip(
                          self.configs.configs, ABMN)):
  
                      if np.all(old_config == new_config):
                          continue
                      # check polarity
                      current_electrodes_are_equal = np.all(
                          old_config[0:2] == new_config[0:2]
                      )
                      voltage_electrodes_are_switched = np.all(
                          old_config[2:4] == new_config[4:1:-1]
                      )
  
                      if(current_electrodes_are_equal and
                         voltage_electrodes_are_switched):
  
                          if len(self.configs.measurements.keys()) > 0:
                              raise Exception(
                                  'need to switch electrode polarity, but ' +
                                  'there are already measurements stored for ' +
                                  'the old configuration!')
                          else:
                              # switch M/N in configurations
                              self.configs.configs[nr, :] = new_config
                      else:
                          raise Exception(
                              'There was an error matching configurations of ' +
                              'voltages with configurations already imported'
                          )
  
          # add measurements to the config instance
          mid_mag = self.configs.add_measurements(
              measurements[:, 2]
          )
          mid_pha = self.configs.add_measurements(
              measurements[:, 3]
          )
  
          self.assignments['measurements'] = [mid_mag, mid_pha]