Add description for the Layer Model screen

[jbphet]

Description for the Photons screen is nearing completion, and the spec is ready for the developers to start implementation of description in the Layer Model screen.

See spec at https://docs.google.com/document/d/1lkpnGpLhbvjRphW5bITHLGyKsj-OGq4WWO98ZM2wKVM/edit?usp=sharing for the details of how this should behave.

[jbphet]

@arouinfar - @jessegreenberg and I have worked through the spec and believe we have covered most everything. I'm assigning this issue to you to start the review. There is one known issue, listed in #375, about the surface and the layers reporting that they've reached equilibrium several times in rapid succession. Please ignore that problem for now, and we will work on it in parallel with your review.

I've published a dev version for you to use for the review so that you're not working with a moving target. Please see https://phet-dev.colorado.edu/html/greenhouse-effect/1.3.0-dev.2/phet/.

[jbphet]

Unassigning myself since I've now resolved the problems related to equilibrium. I'll address issues resulting from the review of the Layer Model screen description as they arise.

[arouinfar]

Layer Model review is complete. I identified a few issues, all tagged above, so closing this issue.

[phet-dev]

Reopening because there is a TODO marked for this issue.

[arouinfar]

@jbphet this issue was automatically reopened due to a TODO, so back to you.

[jbphet]

The TODO said, in part, "Next add the context response for the value change", and it was referring to the sunlight intensity. That was definitely added when the alerter for the Layer Model was created, so it is now safe to simply remove this TODO. Done, re-closing.