[DOC] - Split up & extend filter tutorial(s)

[TomDonoghue]

I think it would be a good idea to re-organize and expand our Filter tutorial, which is currently one tutorial page.

A potential reorganization would split up into: FIR, IIR, and maybe an additional tutorial on the utils / plots available to explore filters.

There are some good resources on guiding the use of filters, that include demos / ideas that we could add in code:

• de Cheveigne et al, 2019: Filters: when, why and how
• Widmann et al 2015: practical digital filter design

The above linked papers have a bunch of specific and practical recommendations. With links to these papers as source, I think we could flesh out our discussion of filters to recommend these points, and demonstrate some of the issues they discuss.

Somewhat related is the idea of a Filter report (#139). Even if this isn't an explicit / separate thing, the tutorials should cover all information that should be reported by a filter, including: filter type, frequency parameters (cutoff frequency), filter order (or length), roll-off or transition bandwidth, passband ripple & stopband attenuation, filter delay (zero-phase, linear phase, etc), causality, and whether it was applied in one or two directions.

Widmann et al compare some software implementations, and also have suggestions for the user (things we should make sure are easy to do). One in particular: examine filter responses by applying them to impulses (compute the impulse response). This is something we can emphasize in the tutorials, consistently showing and doing this.

[TomDonoghue]

This topic is covered in #232, which is being finished to merge, so this issue can be closed now.