Include information about the execution plan and relation in result of an expression

[hadrienk]

The current model only present the result of the execution of a VTL expression as a stream.

Instead, we should use our own representation (probably extending stream) that includes information about how the data is processed, the other operations it is getting data from, etc.

interface VtlPlan {
  Collection<? extends VtlStream<T>> getChildren();
  ? extends VtlStream<T> getParent();
  VtlPipeline getPipeline();
}
class VtlStream<T> extends Stream<T> implements VtlPlan {
  /* ... */
}