Factor out "sky and ground" class, identify possible changes to scenery-phet.SkyNode

[aaronsamuel137]

In FPAF, we are using a work around where another rectangle is required to make the sky node resize without showing whitespace. This functionality should be a part of sky node. Possibly this could be addressed in GradientBackgroundNode and apply to GroundNode as well.

  // add rectangle on top of the sky node. See https://github.com/phetsims/fluid-pressure-and-flow/issues/87
    this.addChild( new Rectangle( -5000, -1000, 10000, 1002, { stroke: '#01ACE4', fill: '#01ACE4' } ) );

    // add sky node
    this.addChild( new SkyNode( -5000, 0, 10000, groundY, groundY ) );

Currently, many sims with sky node exhibit the same bug as #87 including BalancingAct.

See #215

[pixelzoom]

The only sim currently using SkyNode is fluid-pressure-and-flow.

EFAC has its own SkyNode implementation, see phetsims/energy-forms-and-changes#405.

Balancing Act also has its own implementation, OutsideBackgroundNode, see phetsims/balancing-act#128.

There may be other sims that could be using SkyNode, but I have not attempted to identify them.

[pixelzoom]

The workaround identified in https://github.com/phetsims/scenery-phet/issues/79#issue-44468162 is applied in 3 different places in fluid-pressure-and-flow. I've added TODO comments in all 3 places.

I'm not yet convinced that anything needs to be changed in SkyNode. The first thing that should happen is to address the duplication in fluid-pressure-and-flow, and factor out a "sky and ground" class. What happens there can then inform any possible changes to SkyNode.

So... I'm going to transfer this issue to fluid-pressure-and-flow, and change the title.