Convention for naming model-element classes

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For code review #41 ...

PhET convention does not typically use the suffix "Model" for naming model-element classes. So recommended to rename:

• PipeModel => Pipe
• WaterCupModel => WaterCup

IntroModel is fine, since it is PhET convention to name the top-level model "container" with a "Model" suffix.

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Be careful to preserve git history when renaming files. WebStorm/git can be a bit unpredictable about how it handles renames.

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Also by convention, parameters and fields of type PipeModel and WaterCupModel should be renamed. For example, in WaterCup2DNode.ts:

-  private readonly waterCupModel: WaterCupModel;
+  private readonly waterCup: WaterCup;

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Models renamed. Closing.