'pH Scale' implementation notes

All core model computations are in PHModel.

When accessing properties from a PropertySet, I prefer propertySet.fooProperty to propertySet.property( 'foo ' ).

I find that I frequently need to change the rendering order and layout of nodes.
To make that easier, node constructors typically have this structure:

    function MyNode(...) {

        // instantiate nodes

        // set rendering order (addChild)

        // do layout

        // register observers
    }