Removed AggregatedPieces.Metadata Inner Class

Closes #249. Metadata is now handled with a dictionary.
Changed RuntimeError to TypeError in Indexer.__init__()

As requested in closes #251.
Create "test_output" Directory for WorkflowManager

Closes #252.

The word "test" in the directory's name helps clarify that application
developers shouldn't use it.
Allow Setting "offset interval" to None in WorkflowManager

Closes #257.

If you give 0, we set None!