Options extension without setting options

[jessegreenberg]

For #49. I saw this code in DistanceArrowNode.

    _.extend( {
      tandem: Tandem.required
    }, options );

I think this was added for PhET-iO instrumentation. @mbarlow12 @samreid should this be

    options = _.extend( {
      tandem: Tandem.required
    }, options );

[samreid]

The options was incorrect (because it failed to set options = , but I don't think we need to require a tandem for the DistanceArrowNode. I'm inclined to eliminate that statement altogether.

[jessegreenberg]

Sounds good to me, thanks @samreid.

[mbarlow12]

It looks like we're setting a tandem for the label text at the moment. @samreid are you thinking that we don't need any tandems for the DistanceArrowNode, or that we simply don't want to require it.

[samreid]

I'm thinking that we don't need to require the tandem (though we may supply it anyways).

[mbarlow12]

Requirement removed, but we're still passing the tandem through the options.