change model to use 1-based level numbering?

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In phetsims/vegas#73 and phetsims/vegas#75, the consensus was to make vegas use 1-based level numbering. And that 1-based level numbering in the model may be advantageous for PhET-iO, but should be evaluated on a sim-by-sim basis.

Making the requested change for FiniteStatusBar proved to be complicated. It has option levelProperty, which is typically a model Property. And several sims (including this one) use 0-based level numbering in the model, so that the level number can be used as an index into arrays. I investigated changing the semantics of the model Property to be 1-based, but that turned into a large task, touching dozens of files, potentially error prone.

To avoid changing the model semantics, I added a DerivedProperty that handles the mapping from 0-based to 1-based level numbering. In general, it looks like this:

var statusBar = new FiniteStatusBar( ..., {
  ...
  // FiniteStatusBar uses 1-based level numbering, model is 0-based.
  levelProperty: new DerivedProperty( [ model.levelProperty ], 
    function( level ) { return level + 1; } )
} ); 

At some point, evaluate whether this sim's model should be converted to 1-based level numbering. If so, after converting, the above DerivedProperty can be removed, i.e.:

var statusBar = new FiniteStatusBar( ..., {
  ...
  levelProperty: model.levelProperty
} ); 

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The only reason to do this would be for PhET-iO. Deferring until this sim is instrumented.

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There's no reason to do this, not even for PhET-iO. If 1-based numbering is needed for PhET-iO, then instrument the DerivedProperty that's passed to FiniteStatusBar, it is 1-based. See GamePlayNode line 61:

levelProperty: new DerivedProperty( [ model.levelProperty ], function( level ) { return level + 1; } ),