internal SpatialLevelPowerIndexer(SpatialTree <TNode> tree)
 {
     _tree  = tree;
     _power = new SpatialSheetPowerTypes[_tree.SpatialDepth];
 }
 internal SpatialSheetIndexer(SpatialSheetBase <TNode> sheet, SpatialTree <TNode> tree)
 {
     _tree  = tree;
     _sheet = sheet;
 }