Add: slope_sign_change, log_detector and willison_amplitude

[nelsoncardenas]

Features : slope_sign_change, log_detector and willison_amplitude.

This features are popular and useful in time domain analisys for EMG signals. The main reference for this features are: [1] Phinyomark, A., Phukpattaranont, P., & Limsakul, C. (2012). Feature reduction and selection for EMG signal classification. Expert systems with applications, 39(8), 7420-7431.

I'm a newbie to contributing to projects on Github, but I tried to follow the steps in your documentation.

[nils-braun]

Great, @theinem ! Three really interesting features. As @MaxBenChrist mentioned, could you add some unittests for your new feature calculators? We try to test every calculator :-) Also, you new calculators need settings, so you need to add them to the long list in settings.py to the ComprehensiveFCParameters (that will also solve your test failures). Could you also fix the python style issues?

[nelsoncardenas]

As I said in my other PR, I would like to make the improvements you asked for, but for several weeks I have had problems with available time and it will continue like this for several weeks/months.

[nils-braun]

Thanks for the info, @theinem. As mentioned in https://github.com/blue-yonder/tsfresh/pull/825, this is not a problem for us!