changes for frontend of h2o flow to enable TreeVisualization

[msrimat]

[micahstubbs]

will add these comments

[micahstubbs]

another comment from slack

oh @jfohl - sorry, was a question for @Leland. just wondering if anyone is working on the treevis. The answer was: 1) @micah is working on reviewing the flow PR, but blocked on backend changes in H2O to get the tree data, 2) I mentioned there’s a tree interpreter I saw Tom made, which is a cmd line tool, but seems to already be getting the tree data somewhere -> perhaps there’s a way to do tree-vis without changes to H2O.

[micahstubbs]

comment from @ivy_wang ^^

[micahstubbs]

an idea: use the mojo tree viewer @tomkraljevic wrote to QA this.
or we could just use that graph builder directly.

per @tomkraljevic

there were some tricky things to figure out regarding categorical level and N/A reachability calculations as each level is traversed, and rows are eliminated level-by-level.