def save(self):
        tmp_path = self.path + '.tmp'
        with open(tmp_path, 'w') as f:
            f.write(json.dumps(self.body_labels.items()))
        os.rename(tmp_path, self.path)
def mcv_muer(mcv, ax=0):
    """plot rel. permeability vs. B of mcv dict"""
    MUE0 = 4e-7*np.pi
    bi, ur = zip(*[(bx, bx/hx/MUE0)
                   for bx, hx in zip(mcv['curve'][0]['bi'],
                                     mcv['curve'][0]['hi']) if not hx == 0])
    if ax == 0:
        ax = plt.gca()
    ax.plot(bi, ur)
    ax.set_xlabel('B / T')
    ax.set_title('rel. Permeability')
    ax.grid()