[gui] Add RTreeMap

[magnustymoteus]

This Pull request:

Changes or fixes:

This PR adds a new directory treemap in /gui/ with the following features:

• Logic for generating treemap
  • Squarification algorithm is used
• Drawing a treemap on a TVirtualPad
  • This is the non-interactive version without web graphics (interactive JS treemaps will be in a separate PR)
• Importing the treemap given an RNTuple
• CMakeLists.txt that makes this a root library ROOTTreeMap
  • Should this be renamed to ROOTTreeMapv7?
  • Should this file be moved to a separate "CMake integration" PR?
• LinkDef.h so that the treemap structure can be encoded to JSON for use in JS

Checklist:

• tested changes locally
• updated the docs (if necessary)

[github-actions]

Test Results

    21 files      21 suites   3d 11h 45m 42s ⏱️
 3 641 tests  3 486 ✅  0 💤 155 ❌
74 720 runs  74 542 ✅ 23 💤 155 ❌

For more details on these failures, see this check.

Results for commit ab807f9.

♻️ This comment has been updated with latest results.

[silverweed]

LGTM, thanks!

[magnustymoteus]

Some changes I made:

• Removed the clad thing from CMakeLists.txt
• Additional static Import method with RNTupleInspector as argument (needed for the non-web TBrowser)
• General code cleanup (e.g push -> emplace)

[pcanal]

Are we planning any test for these classes? Is there any 'how to use' documentation?

[silverweed]

A brief tutorial would be nice to have indeed.

[pcanal]

Suggested change

	allows for drawing a legend of types of leaf nodes, and see which types occupy how much of the total space.
	*/
	allows for drawing a legend of types of leaf nodes, and see which types occupy how much of the total space.
	Note: this visualization class/technique is independent/unrelated to `TTree`.
	*/