Read in measurement data from a volt.dat file and return electrodes and
    measured resistance.

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