SEG-Y importer LOD generation

The SEG-Y importer needs to be augmented with functionality to create LODs. Since we're writing to an object store we don't want to base the LOD generation on re-downloading all objects in order to generate LODs, so we need to implement an in-memory cache of partially written LOD tiles. If possible this should be implemented as a re-usable utility class that can be used by other importers with similar needs.