set visibleProperty to read-only for some Nodes

[pixelzoom]

2/6/20 design meeting. From #92:

• Visibility of several Nodes are controlled by an associated Property. (LinearGraph, LogarithmicGraph, RatioNode, MoleculesNode, PHMeterNode, others?) Should their visibleProperty be read-only so we can't get into trouble?

Yes, do this.

[pixelzoom]

To make visibleProperty, pass this via options:

        phetioComponentOptions: {
          visibleProperty: {
            phetioReadOnly: true
          }
        }

[pixelzoom]

I made the visibleProperty of the following elements read-only:

moleculeCountNode
ratioNode
linearGraphNode
logarithmicGraphNode
zoomInButton
zoomOutButton
``

I'm going to close this, since this is supposed to be a starting point.  A more comprehensive review will follow.