code: public Extensions getOrCreate(String tableName) {
  
  		Extensions extension = getOrCreate(EXTENSION_NAME, tableName,
  				TileTable.COLUMN_TILE_DATA, DEFINITION,
  				ExtensionScopeType.READ_WRITE);
  
  		return extension;
  	}
docs: Get or create the extension
  
  @param tableName
  table name
  @return extension