Notes from partial review of Layer Model description

[jbphet]

We are partly - and hopefully mostly - through the implementation of description for the Layer Model screen and are reviewing and discussing some of the behavior. Here are some actions that we need to take as a result:

• [x] If there are no active layers, we should leave off the part of the IR object response that says {{More/Fewer}} photons redirecting back to surface.
• [x] For the IR absorbance slider responsive descriptions, they should not be said if there are no layers present.
• [x] Have a separate equilibrium algorithm for the 3rd screen (likely through the use of options), then tweak that algorithm such that what the user sees on the screen matches reasonably well with the time at which stability is announced.

[jbphet]

The items above are addressed, closing. Note that progress on the third item on the list above is being tracked in #378.