Property of Property

[veillette]

While addressing #162, I came across an unusual construct in PeriodTimer.js

    // @public {Property.<pendulum>} flag to identify pendulum
    this.activePendulumProperty = new Property( pendulums[ INIT_PENDULUM_NUMBER ] );

where pendulums is an array of Pendulum, and Pendulum itself is a PropertySet.

so we end up with code such as

self.activePendulumProperty.value.periodTrace.isVisibleProperty.value = self.isRunningProperty.value;