docs: def readin_volt(filename):
      """Read in measurement data from a volt.dat file and return electrodes and
      measured resistance.
      """
      with open(filename, 'r') as fid:
          content = np.loadtxt(fid, skiprows=1, usecols=[0, 1, 2])
          volt = content[:, 2]
          elecs = content[:, 0:2]
      return elecs, volt