Handle debug code with constants rather that commenting it out

[jbphet]

There are a few locations in the sim code where code that is useful for debugging is commented out. PhET prefers to use a pattern where the code is still live, but is deactivated through the use of a constant. This prevents the code from being accidentally deleted during code cleanup, and makes the code more likely to be at least partially maintained as the other code in the file evolves.

For an example of how to apply this technique, please see beers-law-lab/js/concentration/view/DropperNode.js, and search on the string "DEBUG_ORIGIN".

Locations where this was seen as of this writing (this is probably not an exhaustive list):
ParticleWebGLNode.js line 267-272
ParticleWebGLNode.js line 141

[jbphet]

Found during code review, see #29.

[AshrafSharf]

Debug code re-factored to use constants, similar to ab/js/concentration/view/DropperNode.js

[jbphet]

Looks good, closing.