implement Cell:isDate() for unification

src/Spout/Common/Entity/Cell.php

@@ -182,6 +182,14 @@ class Cell
         return $this->type === self::TYPE_STRING;
     }
 
+    /**
+     * @return bool
+     */
+    public function isDate()
+    {
+        return $this->type === self::TYPE_DATE;
+    }
+
     /**
      * @return bool
      */