Make a PersistedProducer interface

[schmmd]

In src/main/scala/org/allenai/pipeline/Producer.scala:

> @@ -125,7 +156,7 @@ trait CachingDisabled extends CachingEnabled {
>    override def cachingEnabled: Boolean = false
>  }
>  
> -class PersistedProducer[T, -A <: Artifact](
> +class PersistedProducer[T, -A <: Artifact] private[pipeline] (

The Producer.persist method returns a PersistedProducer not a simple Producer This really should be changed to return an interface instead of a concrete class. The only additional thing that PersistedProducer exposes is the Artifact that the data was saved to.

[rodneykinney]

I'm addressing this in the branch I'm using to build out some needed functionality exposed by the integration of DBLP matching.

https://github.com/rodneykinney/pipeline/blob/artifact-factory/src/main/scala/org/allenai/pipeline/Producer.scala#L146