Tree visualization

[aprokop]

Per @dalg24:

Related to ornl-cees/DataTransferKit#535 and ornl-cees/datatransferkit#538

Writing down here some of the possible improvements so we can share and I don't forget.

• Add composite visitor to be able to apply multiple visitors within a single traversal. Two options here, inheritance (yuk but easy) or type erasure (harder but fun)
• Implement a visitor that prints out directly the matrices that show what node (col) have been visited for each query (row)
• Put more scripts under version control. More particularly I have in mind the ones that produce Graphviz images of the tree structure and movies that show successive traversals.
• Consider running the scripts in the CI. I am not 100% convinced it's a good idea but just in case here are useful instructions to install missing requirements in our dev/ci container.

[aprokop]

An interesting paper on visualization.