Oops, I was running ~/.python/z_piezo_calib.calibrate_cantilever()
during the experiment, not ~/.python/calibrate_cantilever.something().
I was wondering why it was so difficult to get
~./python/calibrate_cantilever.py working.  I ended up getting

  Cantilever k        : 0.589927 +/- 0.790152 (1.33941)
  photoSensitivity**2 : 0.000190122 +/- 1.86869e-06 (0.00982893)
  T                   : 295.15 +/- 0 (0)
  1/Vphoto**2         : 761447 +/- 1.01986e+06 (1.33937)

out of calibrate_cantilever.py for the first cantilever, but that is
pretty rediculous...  Maybe I'll come back and figure out what's going on
after I check out the z_piezo_calib version...