Use PhET convention for naming Properties related to visibility.

[pixelzoom]

For code review #41 ...

In IntroModel.ts:

    this.isShowingPredictMeanProperty = new BooleanProperty( false, {
      tandem: options.tandem.createTandem( 'isShowingPredictMeanProperty' )
    } );
    this.isShowingMeanProperty = new BooleanProperty( false, {
      tandem: options.tandem.createTandem( 'isShowingMeanProperty' )
    } );
    this.isShowingTickMarksProperty = new BooleanProperty( false, {
      tandem: options.tandem.createTandem( 'isShowingTickMarksProperty' )
    } );

The "isShowing" prefix here does not match PhET/PhET-iO naming conventions.
For Properties that control visibility, the convention is to give them names (and tandem names) of the form {something}VisibleProperty.

So these should Properties and their tandems should be renamed to predictMeanVisibleProperty, meanVisibleProperty, and tickMarksVisiblePropery.

[marlitas]

All property names changed. Closing.

[pixelzoom]

Reopening. I don't see any commits associated with this issue. And I still see the "isShowing" names being used in IntroModel.ts:

  public readonly isShowingPredictMeanProperty: BooleanProperty;
  public readonly isShowingMeanProperty: BooleanProperty;
  public readonly isShowingTickMarksProperty: BooleanProperty;

[marlitas]

Closing. Commits now pushed up.