Dispose or document Bond.dipoleProperty

[zepumph]

from code review #30
This DerivedProperty is never disposed, but it is also not documented that/why dispose is not needed like all other places in the repo.

[pixelzoom]

From implementation-notes.md:

All objects created in this simulation exist for the lifetime of the simulation, so there is no need to call dispose.

But I added a comment:

    // @public dispose not needed, exists for the lifetime of the sim
    this.dipoleProperty = new DerivedProperty( [

[zepumph]

Ok right, just making sure. Thanks for the comment.