Create checkboxGroup for species checkboxes in histograms

[arouinfar]

For #30

The Speed and Kinetic Energy histograms have checkboxes to display the data for a single particle type.

I can see a client wanting to hide both checkboxes, but not just one. @Nancy-Salpepi and I think it would be nice to create a checkboxGroup that contains both heavyParticlesCheckbox and lightParticlesCheckbox. We could then feature checkboxGroup.visibleProperty and uninstrument the individual child checkbox.visibleProperty.

I recognize that request is treading into scope-creep territory, so feel free to veto @pixelzoom.

[pixelzoom]

@arouinfar please review, close if OK.

[arouinfar]

Thanks @pixelzoom, the changes look good on main. During revie, I noticed a possible dynamic layout bug and reported it in #258.

Closing.