Implement TScatter2D (5D plots)

[couet]

Implement TScatter2D.

• New class TScatter2D.
• New example gr019_scatter2d.C superimposing two scatter plot. The color and size take care of color and size ranges of the previous plot.
• Modify TScatter to take care of the color and size ranges of the previous plot. TScatter2D does the same.
• TScatted2D works in linear and log scale.
• Modify gr006_scatter.C to dra two superimposed plots.

[ferdymercury]

Thanks for this!

Fixes #16376

Old attempt: #17185

[linev]

Add checks for fGraph != nullptr.
SavePrimitive can be adjusted later

[github-actions]

Test Results

    22 files      22 suites   3d 20h 38m 58s ⏱️
 3 702 tests  3 702 ✅ 0 💤 0 ❌
79 493 runs  79 493 ✅ 0 💤 0 ❌

Results for commit 79c6fdd.