UP-4262 : toString() in DynamicRenderingPipeline.

Trivial: adds a `toString()` implementation in `DynamicRenderingPipeline` as a step towards enabling better logging.

It's a small step, since the objects included in the `String` do not themselves have friendly `toString()`.  Small steps.

This is in the context of [MyUW's customizations to the rendering pipeline](http://apetro.ghost.io/pipelines-need-valves/), where logging the chosen branch out of a `BranchingRenderingPipeline` is handy.