What is zeroForceIndex and why is it 31?

[jessegreenberg]

For #30. ISLCPullerNode has

    // @private - if in coulomb's law sim, add pusher and zero force images in proper order
    this.zeroForceIndex = null;

    if ( options.attractNegative ) {
      this.pullerPusherImages = pushImages.concat( zeroForceImage ).concat( pullImages );
      this.zeroForceIndex = 31;
    }

@mbarlow12 can you please include documentation about why it is necessary to keep a reference to zeroForceIndex? Also, could zeroForceIndex be set without the magic number? Would this work?

    // @private - if in coulomb's law sim, add pusher and zero force images in proper order
    this.zeroForceIndex = null;

    if ( options.attractNegative ) {
      this.pullerPusherImages = pushImages.concat( zeroForceImage ).concat( pullImages );
      this.zeroForceIndex = pushImages.length - 1;
      this.pullerPusherImages.concat( pullImages );
    }