delete preprocessed GRD products

[johntruckenbrodt]

The recently introduced GRD buffering mechanism (#73) requires preprocessing of up to two neighboring scenes of the target scene. The preprocessed neighbors are kept after processing and with the closing of #75 in #85 the preprocessed product of the target scene is kept as well so that all can be reused for processing other scenes.
Once all neighbors have been fully processed, the preprocessed product of the target scene should be deleted as well. A mechanism needs to be implemented to control this.