added geo centroid aggregation

src/Aggregation/GeoCentroidAggregation.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\ElasticsearchDSL\Aggregation;
+
+use ONGR\ElasticsearchDSL\Aggregation\Type\MetricTrait;
+
+/**
+ * Class representing geo centroid aggregation.
+ *
+ * @link https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-metrics-geocentroid-aggregation.html
+ */
+class GeoCentroidAggregation extends AbstractAggregation
+{
+    use MetricTrait;
+
+    /**
+     * Inner aggregations container init.
+     *
+     * @param string $name
+     * @param string $field
+     */
+    public function __construct($name, $field = null)
+    {
+        parent::__construct($name);
+
+        $this->setField($field);
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function getArray()
+    {
+        $data = [];
+        if ($this->getField()) {
+            $data['field'] = $this->getField();
+        } else {
+            throw new \LogicException('Geo centroid aggregation must have a field set.');
+        }
+
+        return $data;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function getType()
+    {
+        return 'geo_centroid';
+    }
+}