Misleading names for "number of masses"

[pixelzoom]

"Number of mass" seems to actually be the "number of masses per row". Confirm and rename these things:

In NormalModesConstants:

NUMBER_OF_MASSES_RANGE: NUMBER_OF_MASSES_RANGE, //TODO actually number of masses PER ROW

In NormalModesModel:

    // @public {Property.<number>} the current number of visible masses
    //TODO this is actually the number of masses per row. In the 'Two Dimensions' screen, the number of masses is this value squared. https://github.com/phetsims/tasks/issues/1129
    this.numberOfMassesProperty = new NumberProperty( options.numberOfMasses, {