Masses can be dragged behind springNodes and springHangerNode

[Denz1994]

The mass should be the most forward node when dragging. Currently, we can produce this behavior:

[Denz1994]

Consider the below code from BounceScreenView.js

    // TODO: We are reinitializing this.springNodes. Is this the best way to handle this? Ask JO
    // @public {Array.<OscillatingSpringNode>} Initialize a new array of springNodes with a different color selection.
    this.springNodes = model.springs.map( function( spring ) {
      var springNode = new OscillatingSpringNode(
        spring,
        self.modelViewTransform,
        tandem.createTandem( 'oscillatingSpringNode' ), {
          leftEndLength: -10,
          frontColor: new Color('rgb( 227, 153, 221 )'),
          middleColor: new Color('rgb( 185, 0, 169 )'),
          backColor: new Color('rgb( 93, 0, 85 )')
        }
      );
      self.addChild( springNode );
      return springNode;
    } );
    this.springSystemControlsNode.moveToFront();

I will ask @jonathanolson for a closer look at this section during our next collab. We shouldn't have to redeclare our springNodes just because the colors are different in this one instance. Perhaps this is a limit of the current options implementation.

[Denz1994]

The order of adding view components to the scene graph has changed. Please review @arouinfar for the above correction.

The masses should not be able to be dragged behind the springs or controls.

[arouinfar]

@Denz1994 the Stretch and Lab screens look good, but I'm still seeing this on Bounce:

[Denz1994]

Just pushed a fix. Review in master @arouinfar?

[arouinfar]

👍