add dispose method to NumberProperty, https://github.com/phetsims/build-an-atom/issues/156 https://github.com/phetsims/phet-io/issues/1236
add dispose method to NumberProperty, https://github.com/phetsims/build-an-atom/issues/156 https://github.com/phetsims/phet-io/issues/1236

phetsims/axon,phetsims/axon,phetsims/axon