rm obsolete comment
Reformat comment
fix indentation
QD: clarify some variable/method names
QD integration loop: move photons, then interact
Photon absorption.

[#48675313]
rename emitPhoton -> emitPhotonFromAtom
Prototype light source for demo

Energy + angle are hard-wired to those used by the "sunOnGround" model

[#53969913]
Implement turnOnLightSource, turnOffLightSource

May not be the best API for final light source implementation.

[#53969913]
Convert alternate form of MML energy levels

[#53959593]
add light source to interactive

[#48675313]
Make sure photon angles are in range [-pi, pi)
check viewPhoton ids too

[#48675251]