Replace string literals with enumerations

[pixelzoom]

Use StringEnumerationProperty or EnumerationProperty for the Properties shown below. This should be done before publishing a PhET-iO version.

// ColorVisionModel

    this.headModeProperty = new StringProperty( 'no-brain', {
      validValues: [ 'brain', 'no-brain' ],
      tandem: tandem.createTandem( 'headModeProperty' )
    } );

// SingleBulbModel

    this.lightTypeProperty = new StringProperty( 'colored', {
      validValues: [ 'white', 'colored' ],
      tandem: tandem.createTandem( 'lightTypeProperty' )
    } );

    // @public {Property.<string>} indicates solid beam vs individual photons
    this.beamTypeProperty = new StringProperty( 'beam', {
      validValues: [ 'beam', 'photon' ],
      tandem: tandem.createTandem( 'beamTypeProperty' )
    } );

[Luisav1]

Deferring this issue since we're not going to republish with PhET-iO for the upcoming publication.