RealMoleculesScreenView is doing unnecessary work

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We're releasing Molecule Polarity 1.0.0 without an implementation of the Real Molecules screen, see #27. But in the meantime, it is still doing a lot of unnecessary work, which will impact startup time.

RealMoleculesScreenView constructor currently looks like this:

function RealMoleculesScreenView( ... ) {

  ScreenView.call( ... );

  // create a whole bunch of Nodes and add them to rootNode

  if ( MPQueryParameters.realMolecules ) {
    this.addChild( rootNode );
  }
  else {
    this.addChild( new UnderDevelopmentPlane( this.layoutBounds ) );
  }
}

Better would be:

function RealMoleculesScreenView( ... ) {

  ScreenView.call( ... );

   if ( !MPQueryParameters.realMolecules ) {
    this.addChild( new UnderDevelopmentPlane( this.layoutBounds ) );
  }
  else {
    // create a whole bunch of Nodes and add them to rootNode
    this.addChild( rootNode );
  }
}

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Creating the model (RealMoleculesModel) is lightweight, so it's OK to do that and not use it. The significant cost is in creating the view.

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Done, tested with and without ?ea&realMolecules.

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@zepumph FYI, since you're in the process of code review.