change model to use 1-based level numbering

[pixelzoom]

In https://github.com/phetsims/vegas/issues/73 and https://github.com/phetsims/vegas/issues/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
} ); 

[pixelzoom]

Here's this sim's usage of FiniteStatusBar, in BAAGameScreenView:

var scoreboard = new FiniteStatusBar( {
  ...
  levelProperty: gameModel.levelProperty,
  ...
  levelVisible: false,
  ...
} );

Because the level is not visible, there's no reason to provide option levelProperty. So rather than add the DerivedProperty adapter described above, I'm going to simply remove the levelProperty option.

[jbphet]

The change looks reasonable, and I played a level of the game and everything looked fine. Thanks, closing.