Change requests for graphingQuadratics.standardFormScreen.view.rootsNode

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For #137 and #139...

graphingQuadratics.standardFormScreen.view.rootsNode (RootsNode.js):

• When there is 1 root, rightCoordinatesProperty should have the same value as leftCoordinatesProperty (it's currently null).
• rightRootNode should still be invisible if there is 1 root.
• rightCoordinatesProperty should have same initial values as leftCoordinatesProperty.
• Verify what happens with leftRootNode and rightRootNode if you set visibleProperty=true when there are no roots.

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Summary of changes in the above commits:

• When there is 1 root, leftCoordinatesProperty and rightCoordinatesProperty are the same.
• leftCoordinatesProperty and rightCoordinatesProperty are both intiialzed to (0,0)
• Updated phetioDocumentation for leftCoordinatesProperty and rightCoordinatesProperty
• Updates phetioDocumentation for leftRootNode and rightRootNode

Testing in Studio:

• Inspected leftCoordinatesProperty and rightCoordinatesProperty
• Inspected phetioDocumentation
• Verified that with no roots, setting visibleProperty=true for leftRootNode and rightRootNode does not result in anything being displayed. Note that visibleProperty will automatically change when the roots change, so changing visibleProperty does not result in a persistent change. So we might consider making visibleProperty phetioReadOnly: true.

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@amanda-phet ready for review in master or 1.2.0-dev.1.

Note this possible change in #146 (comment):

... Note that visibleProperty will automatically change when the roots change, so changing visibleProperty does not result in a persistent change. So we might consider making visibleProperty phetioReadOnly: true.

[amanda-phet]

Note that visibleProperty will automatically change when the roots change, so changing visibleProperty does not result in a persistent change. So we might consider making visibleProperty phetioReadOnly: true.

graphingQuadratics.standardFormScreen.view.rootsNode.leftRootNode.visibleProperty already appears to be read only, so I'm not sure I follow. We do what this to be read only, so it seems to be working correctly.

I confirmed that the roots are behaving as requested, which is great!