Use inheritance instead of composition for WaterLevelTriangleNode

[pixelzoom]

For code review #41 ...

WaterLevelTriangleNode is effectively a slider (with a custom thumb and invisible track). But it currently extends Node, and it's name does not identify it as a slider. Using composition instead of inheritance here is not advantageous, and has disadvantages:

(1) It's a slider, and calling it something else is confusing.

(2) PhET-iO (and Studio) wants it to look like a slider. You're having to make it look like a slider in the tandem hierarchy by defining const sliderTandem, and you won't need to do that if it extends VSlider.

(3) For a11y, it should be a slider. This will become more important as other a11y features are added.

(4) If it extends VSlider, you won't need to override dispose.

So I recommend:

• rename WaterLevelTriangleNode to WaterLevelSlider
• reimplement so that WaterLevelTriangleNode extends VSlider
• rename this.waterLevelTriangle to this.waterLevelSlider in WaterCup3DNode.ts, and change its tandem name to match

[pixelzoom]

Similar comments about PredictMeanNode:

**
 * Representation for the draggable predict mean line
...
export default class PredictMeanNode extends AccessibleSlider( Node, 0 )

Nothing about the name of PredictMeanNode tells me that it's a line or a slider. Suggested to rename to PredictMeanLine, and document that it behaves like a slider for a11y purposes.

[marlitas]

WaterTriangle now inherits from VSlider. Closing.