added Row::getCellAtIndex method

tests/Spout/Common/Entity/RowTest.php

@@ -70,6 +70,18 @@ class RowTest extends \PHPUnit\Framework\TestCase
         $this->assertEquals(2, $row->getNumCells());
     }
 
+    /**
+     * @return void
+     */
+    public function testGetCellAtIndex()
+    {
+        $row = new Row([], null);
+        $cellMock = $this->getCellMock();
+        $row->setCellAtIndex($cellMock, 3);
+        $this->assertEquals($cellMock, $row->getCellAtIndex(3));
+        $this->assertNull($row->getCellAtIndex(10));
+    }
+
     /**
      * @return void
      */