Expand TemperatureDescriber's capabilities or narrow its scope

[jbphet]

While working on the Layer Model screen, I found that I needed to add support for ground temperature values below the previously established minimum. This is mostly done, and about to be committed, but the code in TemperatureDescriber is now defining a value for the range that is only applicable to the first two screens.

@jessegreenberg - I'm not sure whether it would be best to narrow the scope of this, perhaps calling it LandscapeTemperatureDescriber, or generalizing it to work for arbitrary temperature ranges. What do you think?

[jbphet]

Deferring until description becomes a priority again.

[jbphet]

Undeferring since the description feature is a priority for this iteration. Assigning to @jessegreenberg to respond to the question raised in the initial comment.