Update 3D contours

[ffreyer]

Description

Mildly related to #5194

This pr updates contour(..., ::Array{Float, 3}) to rely on the compute graph and introduced some changes to the colormapping:

• [change] colorrange now defaults to the extrema of levels rather than the extrema of the given data
• [fix] colormap used to always generate colors for the full value range minimum(volumedata) .. maximum(volumedata), but report colorrange as its limits to volume. This made any 3D contour with a non default colorrange incorrect. Now the colormap generates for slightly padded colorrange (to include a transparent min and max color) which gets passed down correctly.
• [fix] isorange is now a setable attribute again
• [change] default isorange no longer assumes uniform levels. Instead it's generated based on the minimum distance between levels
• [internal change] The number of colors in the colormap is no longer based on the number of levels. Instead it aims to resolve the padded colorrange in steps of isorange, so that each iso surface has at least one non-transparent color mapped to it
• [change] Colorbar now looks at colormap alpha as a binary so that transparent contours still have a readable Colorbar

By example:

r = range(-1.2, 1.2, 21)
data = Array{Float32, 3}(undef, 21, 21, 21)
for (i, x) in enumerate(r)
    for (j, y) in enumerate(r)
        for (k, z) in enumerate(r)
            data[i, j, k] = norm(Vec3f(x,y,z))
        end
    end
end
f = Figure(size = (400, 1200))
contour(f[1, 1], data, alpha = 0.1)
contour(f[2, 1], data, alpha = 0.1, levels = 0:0.2:1)
contour(f[3, 1], data, alpha = 0.1, colorrange = (0, 1))
contour(f[4, 1], data, alpha = 0.1, levels = 0:0.2:1, colorrange = (0, 1))
f

before	after

• 1, 2 show off different colorrange defaults
• 3, 4 show off the colormap - colorrange missmatch

TODO:

• cleanup Colorbar for 3D contour
• reconsider default colorrange (should it continue to look at the full value range instead?)
• reconsider color sample density (is 1-2 colors per sample enough?)

Type of change

• Bug fix (non-breaking change which fixes an issue)

... maybe breaking due to change in colorrange defaulting?

Checklist

• Added an entry in CHANGELOG.md (for new features and breaking changes)
• Added or changed relevant sections in the documentation
• Added unit tests for new algorithms, conversion methods, etc.
• Added reference image tests for new plotting functions, recipes, visual options, etc.

[MakieBot]

Benchmark Results

SHA: 2db05a7fc471ccffddac2f9dbae0bb4143f66619

Warning

These results are subject to substantial noise because GitHub's CI runs on shared machines that are not ideally suited for benchmarking.

[SimonDanisch]

Is this ready?