Changes to "zero"

[amanda-phet]

Since zero is neither positive nor negative, we need a few special behaviors at 0.

Elevation scene

• [x] When an item is at 0m, the absolute value text should say "sea level" instead of "0m above sea level."
• [x] If possible, the items at 0m should maintain the state they were in as they approach 0. So, if dragging up, a bird might be diving at -1m and at 0m, is still diving. If dragging downward, the bird might be flying at 1m and at 0m, is still flying. Can we do this?

Bank scene

• [x] When a bank is at $0, the absolute value text should continue to say "balance of $0" but the text should be black, not green.

Temperature scene (This is very difficult to produce, but we need a way to handle it should it occur.)

• [x] When a thermometer is at 0º, the absolute value text should say "0º" and should not indicate above or below 0.

[jbphet]

@amanda-phet - I've implemented these changes on master, please check them out and let me if they look okay.

[amanda-phet]

All look great to me. Thanks so much.