Clicking the Button in AddModelGroup really adds the models on the right to the group

Clicking the Button in AddModelGroup really adds the models on the right to the group which makes them show up in the graph.

Verify this by clicking the add model group button.

• select one of the 2-10. Model listed.
• move it to the right
• click add
• the graph should now include the models (hiding the model group with the eye symbol should still work

Additionally, I adjusted the modelsSlice because I was working with it in this task and I corrected the tests that relied on the "old" structure of keeping a separate list of models and groups.