Provide a Savitzky-Golay filter

[niksirbi]

Is your feature request related to a problem? Please describe.
As @sfmig reminded me, this type of filter is very popular for smoothing pose tracks. We should have it as a standard filtering option in movement.

Describe the solution you'd like
Luckily, it's implemented in scipy (see more in-depth explanation here), so it should be simple to apply it to our xarray objects.

Additional context
That said, this filter has disadvantages and alternatives are being proposed.