Consider keeping track of number of particles on phase changes screen

[ariel-phet]

As I flip between the various atoms/molecules if I have pumped in some particles, that information is lost.

So for instance, if I begin on Neon, pump some particles in, flip to Oxygen, and then back to Neon, the particles I have pumped in go away.

If someone explores each atom/molecule one at a time and does not flip back this behavior will not be noticed, but it does seem a bit incongruous since the "model" temperature stay consistent (so if one atom/molecule is set to its triple point for instance, all atoms and molecules are at their triple point when flipping between choices).

This is nothing that needs to be addressed currently, but I wanted to leave it as an open question for discussion next time we visit this sim, so no assignee, no priority.

[jbphet]

@ariel-phet - okay to mark this as deferred?