Add new primitive: Butterworth filter

Description

A low pass filter (in this example a butterworth filter) for the preprocessing of time series data.

The outcome of such a filter should be similar to the moving aggregations, but the number of samples will not be decreased and therefore might improve the performance of the pipeline.

It takes an array of the data, that should be filtered, as input and returns another filtered array.

What I Did

I started implementing this primitive for testing purposes in the butterworth branch on my fork, which you can check out.

Concretely, I added a Primitive JSON file and a custom function in timeseries_preprocessing.py.

Any feedback on the primitive itself and the implementation would be highly appreciated.