moving constantRadiusProperty to ISLCModel

[mbarlow12]

@samreid I noticed your TODO in GFLBModel.js regarding moving the constant radius to ISLCModel. I agree that this (along with the baseColors) would simplify a lot of the code in all the ISLC sims. My concern is that, since it's instrumented or PhetIO, offering that API in Coulomb's Law doesn't really make any sense as there's no way to update the radius were that property set to true.

Is there a way to conditionally set it as a read-only property in CL and have it fully instrumented in GFL and GFLB?

[samreid]

Sure, you can pass phetioReadOnly as an option to the Property. But I noticed the general flow is:

    this.constantRadiusProperty = new Property( false, {
      tandem: tandem.createTandem( 'constantRadiusProperty' ),
      phetioType: PropertyIO( BooleanIO )
    } );

 // ...
    var mass1 = new Mass( value1, position1, valueRange, density, this.constantRadiusProperty, baseColor1, tandem.createTandem( 'mass1' ), massOptions );
    var mass2 = new Mass( value2, position2, valueRange, density, this.constantRadiusProperty, baseColor2, tandem.createTandem( 'mass2' ), massOptions );

    ISLCModel.call( this, ISLCConstants.G, mass1, mass2, leftBoundary, rightBoundary, tandem, {
      snapObjectsToNearest: GravityForceLabBasicsConstants.MASS_POSITION_DELTA,
      minSeparationBetweenObjects: 200 // in meters
    } );

So it wouldn't be a trivial change to move this Property to ISLCModel, because the Property is needed before creating the masses, and the masses are needed before creating the ISLCModel. So we may wish to leave this as it is (or it would require a more invasive change).

[mbarlow12]

Good point—I'd forgotten about that element. I think I'll opt for leaving as is.

@samreid unless you have objections, I'll close and remove the TODO for now.