Document (and/or implement better) colormaps with 3d plots

[kcrisman]

Perhaps ironically, in #12212 we now have a pretty nice syntax for using colormaps with an arbitrary function (scaled to 1) with parametric and implicit plots, but not "regular" 3d plots! We just have Harald's (edited by Jason) example

sage: var('r v')
sage: cmsel = [colormaps['autumn'](i) for i in sxrange(0,1,0.05)]
sage: p = plot3d(0.2*(r**2 + v**2) + cos(2*r)*sin(2*v),(r,-2,2), (v,-2,2), adaptive=True, color=cmsel, plot_points=10, opacity=0.9)
sage: p2 = sphere((0,0,0),1,color='black',opacity=0.5)
sage: (p+p2).show(aspect_ratio=(1,1,1), figsize=[7,3])

which isn't ideal.

We need to document this, or (much better) use the same syntax, and maybe even enable it even when adaptive=False.

See also ​this ask.sagemath question and ​this one and ​this one.

[chapoton]

I just had a first look at the question.

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New commits:

​8fc72c3

trac #17660 first exploration of the problem

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​fbd53f8

Merge branch 'u/chapoton/17660' in 8.0.b9

[chapoton]

one example added in #23859