Implement `aspect_ratio = :equal` for 3D Plots

[BeastyBlacksmith]

I started this some time ago with @rafael-guerra-www and got it wrapped up now.

Fix #1949

It's definitely better than having nothing, but it also always produces a square box, so the example from #1949 now looks like

So bounds could be tighter, but at least you get spheres.

[BeastyBlacksmith]

One issue here is that this doesn't work for backends that don't use a square box for 3D Plots like pgfplotsx.

[lmiq]

Any chance of merging this soon? Anything that has to be done?

[BeastyBlacksmith]

hmm.. its not a universal solution. So there are definitely cases where this doesn't work. On the other hand it may be better than the current status quo..

[rafael-guerra-www]

Hello. This doesn't seem to be implemented yet. Please check this recent thread in discourse and the MWE.