Proof of concept Recipes for Plots

[csimal]

Hi there,

I've implemented spectralportrait as a Plots Recipe as a proof of concept of how this could be done.
This would allow having only one implementation for Plots that works for every backend.

Example use:

using Plots, Pseudospectra, LinearAlgebra
# pyplot() use any Plots backend

n = 150
B=diagm(1 => fill(2im,n-1), 2 => fill(-1,n-2), 3 => fill(2,n-3), -2 => fill(-4,n-2), -3 => fill(-2im, n-3))
Pseudospectra.PSAPlots.spectralportrait(B)

I'd be happy to help implement the rest of the plotting functions as recipes if needed.

[csimal]

Note: It goes without saying that this PR is not really meant to be merged into the main branch. It's just a preliminary.

[RalphAS]

Thanks! When I wrote most of this (several years ago) I found working with Recipes very frustrating and didn't pursue them, but if you're able to get them to handle the content here, I would be glad to replace some of my plotting hackery.

[csimal]

I've gotten most of the way through basic recipes. I'll add tests for these next.

Seeing as Makie is much better for interactivity, I think it might be better to leave that out for the Plots support (especially as it makes the code at lot less gory)