Link `model.histogramsModel` under each `histogramNode`

[arouinfar]

For #30

It would be helpful to link the histogram data, found under model.histogramsModel to the histogramNode view element.

My initial instinct was to add a link to the specific histogram, but it seems like the histogramsModel.kineticEnergy and histogramsModel.speed are just organizational and not instrumented things. (Nothing shows up in the Element Panel when I select them in the tree.)

If I'm mistaken and we can link to histogramsModel.kineticEnergy and histogramsModel.speed, let's do that. Otherwise, linking to histogramsModel is perfectly fine.

@pixelzoom please

• [x] Add a link to the model data for each histogramNode
• [x] So that these newly-added links appear in the Featured tree, make histogramNode and the target of the links phetioFeatured: true.

[pixelzoom]

If I'm mistaken and we can link to histogramsModel.kineticEnergy and histogramsModel.speed, let's do that. Otherwise, linking to histogramsModel is perfectly fine.

Linking to histogramsModel.kineticEnergy and histogramsModel.speed would require some relatively major code changes. So I linked to histogramsModel, and added phetioFeatured: true where requested.

@arouinfar please review, close if OK.

[arouinfar]

Thanks @pixelzoom! Looks good in main, closing.