Oops, Calibrator.save_results should be a static method.

Oops, Calibrator.save_results should be a static method.

Without this fix, you get errors like:

  Traceback (most recent call last):
    File "/usr/bin/calibcant-calibrate.py-2.7", line 46, in <module>
      sys.exit(main(sys.argv[1:]))
    File "/usr/bin/calibcant-calibrate.py-2.7", line 36, in main
      k,k_s,data = calibrator.calibrate(filename=filename)
    File "/usr/lib/python2.7/site-packages/calibcant/calibrate.py", line 495, in calibrate
      spring_constant=k, spring_constant_deviation=k_s, **data)
  TypeError: save_results() got multiple values for keyword argument 'filename'