Returns an S2CellUnion that covers (GetCovering) or is contained within
(GetInteriorCovering) the given region and satisfies the current options.
Note that if options().min_level() > 0 or options().level_mod() > 1, the
by definition the S2CellUnion may not be normalized, i.e. there may be
groups of four child cells that can be replaced by their parent cell.
Returns an S2CellUnion that covers (GetCovering) or is contained within (GetInteriorCovering) the given region and satisfies the current options.
Note that if options().min_level() > 0 or options().level_mod() > 1, the by definition the S2CellUnion may not be normalized, i.e. there may be groups of four child cells that can be replaced by their parent cell.