INDEX(): fix getting row and col numbers from references (#239)

Allow to use cell references as row and column in function INDEX(). Eg:

```
=INDEX(A1:B5, A9)
```

src/PhpSpreadsheet/Calculation/LookupRef.php

@@ -601,6 +601,9 @@ class LookupRef
      */
     public static function INDEX($arrayValues, $rowNum = 0, $columnNum = 0)
     {
+        $rowNum = Functions::flattenSingleValue($rowNum);
+        $columnNum = Functions::flattenSingleValue($columnNum);
+
         if (($rowNum < 0) || ($columnNum < 0)) {
             return Functions::VALUE();
         }

tests/data/Calculation/LookupRef/INDEX.php

@@ -73,4 +73,18 @@ return [
         2,
         2,
     ],
+    [
+        4, // Expected
+        // Input
+        [
+            '20' => ['R' => 1, 'S' => 3],
+            '21' => ['R' => 2, 'S' => 4],
+        ],
+        [
+            '21' => ['R' => 2],
+        ],
+        [
+            '21' => ['R' => 2],
+        ],
+    ],
 ];