Implement the photons' undetermined state after crossing the beam splitter

[AgustinVallejo]

Working on the feature that supports undetermined photons: When they pass through the beam splitter, each photon should have information of its two possible positions, and in the view, their opacity should be determined via its probability.

I'm thinking of adding a possibleStates array to photons to store all the superposed states (position and probability) and get the sprites to listen to that array. Haven't figured yet how to add it to phetio state. Will take a look at other simulations for now and add the patch here once it's in a better shape

[AgustinVallejo]

Closing because #65 already existed