Updated fit_betasbp_cut.py to v0.5.0, supported parameter bounds.
Updated fit_betasbp_cut.py to v0.5.0, supported parameter bounds.

* added 'fit_model_bounds' using 'scipy.optimize.minimize' to
  perform function minimization with bounds
* split the data cut section to function 'cut_data'
* added argument 'options' to 'fit_model_bounds'

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