remove unused import
Improve several aspects of fit_component:

* Fix parameters instead of disabling components
* Backup the "free state" of the components to leave the model configuration intact after calling the method
* Add a fit button instead of fitting when the slider parameters change
* Use the set_signal_range method to select the spectra range where to fit instead of cropping the spectrum
* Add docstring
Merge branch 'gui_fit_model_components' of github.com:hyperspy/hyperspy into gui_fit_model_components