Separate the phase diagram out of PhaseDiagramAccordionBox

[jbphet]

PhaseDiagramAccordionBox builds the whole phase diagram inside of an accordion box, which works, but isn't a very good separation of concerns. I should pull the phase diagram into its own node which is then constructed and included in the accordion box.

[jbphet]

I did some work on this and improved the situation, but in the process I realized that there is way too much coupling between the PhaseChangesScreenView and the phase diagram. This could be refactored such that the phase diagram has all the knowledge necessary to draw the diagram as long as it has a reference to the model. As it stands now, the PhaseChangesScreenView instance sets the marker position, and it has to know a lot about the diagram to do so.

I started on this refactor, but it was taking more time than I could reasonably justify. I'm going to leave it at this for now, and if there is ever a need to do more generalization of this diagram, I or someone else will need to take it from here.

[jbphet]

Unassigning, since it is unlikely that I'll have any time to work on it in the foreseeable future.

[jbphet]

When this is done, care must be taken to make sure that it does not alter the phet-io API.