Replace x and y with position

[pixelzoom]

For code review #41 ...

PipeModel and WaterCupModel (and their IOTypes) have options and fields named x and y. For example in PipeMode.ts:

type StateObject = {
  x: number;
  y: number;
}

type SelfOptions = {
  //REVIEW non-obvious options are supposed to be documented where defined, and these are not obvious to the reviewer
  //REVIEW if x and y are the pipe's position, then position: Vector2 is preferable
  x: number;
  y: number;
  isOpen?: boolean;
};
...
  public readonly x: number;
  public readonly y: number;
``` 

While `x` and `y` are undocumented, I gather that they are the position of the model element.  In that case `position: Vector2` is generally preferred. 

Also note also from the CRC:

> - [ ] PhET prefers to use the term "position" to refer to the physical (x,y) position of objects. This applies to both brands, but is more important for the PhET-iO API. See https://github.com/phetsims/phet-info/issues/126

[marlitas]

x & y replaced with position. Closing.