Make getConcreteSheetIterator() protected (#334)

src/Spout/Reader/AbstractReader.php

@@ -45,7 +45,7 @@ abstract class AbstractReader implements ReaderInterface
      *
      * @return \Iterator To iterate over sheets
      */
-    abstract public function getConcreteSheetIterator();
+    abstract protected function getConcreteSheetIterator();
 
     /**
      * Closes the reader. To be used after reading the file.

src/Spout/Reader/CSV/Reader.php

@@ -131,7 +131,7 @@ class Reader extends AbstractReader
      *
      * @return SheetIterator To iterate over sheets
      */
-    public function getConcreteSheetIterator()
+    protected function getConcreteSheetIterator()
     {
         return $this->sheetIterator;
     }

src/Spout/Reader/ODS/Reader.php

@@ -53,7 +53,7 @@ class Reader extends AbstractReader
      *
      * @return SheetIterator To iterate over sheets
      */
-    public function getConcreteSheetIterator()
+    protected function getConcreteSheetIterator()
     {
         return $this->sheetIterator;
     }

src/Spout/Reader/XLSX/Reader.php

@@ -80,7 +80,7 @@ class Reader extends AbstractReader
      *
      * @return SheetIterator To iterate over sheets
      */
-    public function getConcreteSheetIterator()
+    protected function getConcreteSheetIterator()
     {
         return $this->sheetIterator;
     }