Review phet-io for screens 1-3

[jbphet]

Under #63 the initial phet-io tree was implemented for the first three screens (screen 4 is still undergoing major design iterations). We've also had a few real-time reviews with @arouinfar, @AgustinVallejo, and myself. The feedback from these meetings has been folded in and is now ready for @arouinfar to review more formally.

A dev version has been published for this review and can be found at https://phet-dev.colorado.edu/html/quantum-measurement/1.0.0-dev.14/phet-io/. @arouinfar - please go through it and note in this issue any changes you recommend.