use NumberProperty

[pixelzoom]

Related to code review #247

There are multiple places where Property should be replaced with NumberProperty. I count ~24. Search for "new Property" to locate them. This is especially important if this is a PhET-iO release.

For example, in Beaker:

      // @public {Property.<number>} - fluid level in beaker, should only be set in sub-types
      this.fluidLevelProperty = new Property( EFACConstants.INITIAL_FLUID_LEVEL );

      // @public (read-only) {Property.<number>} - temperature of fluid in beaker
      this.temperatureProperty = new Property( EFACConstants.ROOM_TEMPERATURE );

[chrisklus]

Ready for review

[pixelzoom]

👍 closing.