Add the ability to specify per-item properties in a radio-button group.

[pixelzoom]

In calculus-grapher, we need the ability to set options for specific radio buttons in a group. Specifically, we need to set the visible option of each radio button based on some criteria, which can later be changed via PhET-iO.

This is not currently supported by RectangularRadioButtonGroup or AquaRadioButtonGroup. My plan is to add options to both RectangularRadioButtonGroupItem and AquaRadioButtonGroupItem. (This is identical to how this feature is supported by VerticalCheckboxGroupItem.)

[pixelzoom]

Done in the above commits. I would have preferred to name these new options RectangularRadioButtonGroupItem.radioButtonOptions and AquaRadioButtonGroupItem.radioButtonOptions. But I went with options, to match VerticalCheckboxGroupItem.options, which I did not have time to attempt to change.

@samreid since you worked on Gorups/Items recently, could you please review? Close if OK.

[samreid]

I reviewed the commits and everything seemed reasonable, closing.