Implement ASAP() smoother

From using Datadog, I've come to find their autosmooth()-function very handy when dealing with "spiky" data series.

Their implementation (blog post) is inspired by the ASAP smoother (Automatic Smoothing for Attention Prioritization; homepage, PDF paper).

While it is not perfect for every use-case, I have found it to work extremely well compared to hand-tuning some "average over X minutes" until a pattern actually emerges, yet it doesn't hide actual spikes.

Example from the ASAP homepage: