Add dedicated RowIteratorInterface and SheetIteratorInterface

src/Spout/Reader/ReaderInterface.php

@@ -21,7 +21,7 @@ interface ReaderInterface
      * Returns an iterator to iterate over sheets.
      *
      * @throws \Box\Spout\Reader\Exception\ReaderNotOpenedException If called before opening the reader
-     * @return \Iterator To iterate over sheets
+     * @return SheetIteratorInterface To iterate over sheets
      */
     public function getSheetIterator();