tape.EnsembleFrame Repartition Doesn't Track Parent Ensemble

[wilsonbb]

Currently tape.EnsembleFrame.repartition returns an EnsembleFrame with an empty ensemble field. This results in EnsembleFrame.update_ensemble being a no-op instead of updating the parent ensemble to have the resulting frame.

This can be easily fixed by overriding repartition in the _Frame parent class and calling _propagate_metadata