Model - nodesByName - use id if name is undefined

[lilleyse]

model.nodeTransformations references nodes by their "name" property, which won't work if the node does not have a name. Since its a bit ambiguous what is the id vs. the name, this PR allows the model._runtime.nodesByName to be the id if the name is undefined.

A different approach for the same problem is modifying getNode to look inside nodesByName first and then nodes. Same with getMesh and getMateral.

This came up on the forum: https://groups.google.com/forum/#!topic/cesium-dev/Eqttp9vcR90

[pjcozzi]

In glTF, name and id are separate concepts. name is a user-defined application-facing identifier, id is a glTF internal identifier to link together parts of the asset.

I don't think we should mash them together here or anywhere.

In #927, the plan was

[ ] Add ById versions of functions like getNode?

We close close this for now or, if you think this is timely, @shehzan10 could take the ById approach.

[lilleyse]

Fine with me to close now, this was just a proof of concept.