Enhance data smoothers

Fix several issues in the 1D data smoothers:

* Fix smooth_lowess that was broken.
* Add tests.
* Improve the docstrings.
* Replace in-house savitzky-golay filter by the scipy one.
* Replace in-house lowess filter by the statsmodels one.
* Add +/- buttons to savitzky-golay UI to easily increase the window size.
* Use `Signal.map` to iterate over the signal.