added nested aggregation

Aggregation/NestedAggregation.php

+<?php
+
+/*
+ * This file is part of the ONGR package.
+ *
+ * (c) NFQ Technologies UAB <info@nfq.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace ONGR\ElasticsearchBundle\DSL\Aggregation;
+
+use ONGR\ElasticsearchBundle\DSL\Aggregation\Type\BucketingTrait;
+
+/**
+ * Class representing NestedAggregation.
+ */
+class NestedAggregation extends AbstractAggregation
+{
+    use BucketingTrait;
+
+    /**
+     * @var string
+     */
+    private $path;
+
+    /**
+     * Return path.
+     *
+     * @return string
+     */
+    public function getPath()
+    {
+        return $this->path;
+    }
+
+    /**
+     * Sets path.
+     *
+     * @param string $path
+     */
+    public function setPath($path)
+    {
+        $this->path = $path;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function getType()
+    {
+        return 'nested';
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function getArray()
+    {
+        if (count($this->aggregations) == 0) {
+            throw new \LogicException("Nested aggregation `{$this->getName()}` has no aggregations added");
+        }
+
+        return ['path' => $this->getPath()];
+    }
+}