Add function to set all parameters in a component to free or not free

- component.set_parameters_free:
Sets either all the parameters in a component free, or just the ones
with the same name as supplied in a list.
- component.set_parameters_not_free:
Same as set_parameters_free, but opposite.
Small change to input handling in set_parameter_free/not_free

As suggested by Francisco
DOC: fix pdf link
DOC: fix pdf link (correct typo)
EHN: background for stack of image.

Test in plot works but not plotting
Add Signal.__setitem__
BUG-FIX: the image plots were not printing the intensity at the cursor position
BUG-FIX: utils.estimate_variance_parameters mask parameter was not working as expected
BUG-FIX: ScalableFixedPatter was raising an error when setting the spectrum
BUG-FIX: Model.extend was raising an error because _create_arrays was being called before setting the _axes_manager attribute. Also disable the unsupported insert method
BUG-FIX: typo was causing an Exception
Disable messages in Model fit_background and fit_edge
Rename Signal.estimate_variance to estimate_poissonian_noise_variance
Merge branch 'master' into parameter_set_free_unfree
Fix documentation in component.set_parameter_free and set_parameter_not_free
Add functions to set parameters in components in models to free or not
Add test for parameter.set_parameters_free and set_parameters_not_free
Add tests for m.set_parameters_free/not_free
BUG-FIX: plotting a signal with negative axis scale was raising an exception due to negative aspect ratio
Fixes #80