Added toFlatRegion() to Region.
Added support for ColumnVisitor to work in batches.
Fixed cancellation of operations in CallbackExecutor not calling Operation.cancel().
EditSessions for operations are now managed with a wrapper operation.
Changed //naturalize //replace //center //overlay to use operations.
Minor formatting change in NaturalizeArea.
Fixed OverlayBlocks not actually overlaying blocks but replacing the top one.