Provide connector method for term extraction

* VuFindSearch/Backend/Solr/Connector.php (terms): New method. Extract
  index terms.
* VuFindSearch/Backend/Solr/Backend.php (terms): Use connector method.

module/VuFindSearch/src/VuFindSearch/Backend/Solr/Backend.php

@@ -301,7 +301,7 @@ class Backend implements BackendInterface, MoreLikeThis, RetrieveBatchInterface
         $params->set('terms.lower.incl', 'false');
         $params->set('terms.sort', 'index');
 
-        $response = $this->connector->query('term', $params);
+        $response = $this->connector->terms($params);
         $terms    = new Terms($this->deserialize($response));
         return $terms;
     }

module/VuFindSearch/src/VuFindSearch/Backend/Solr/Connector.php

@@ -220,6 +220,18 @@ class Connector
         return $this->select($params);
     }
 
+    /**
+     * Extract terms from a SOLR index.
+     *
+     * @param ParamBag $params Parameters
+     *
+     * @return string
+     */
+    public function terms(ParamBag $params)
+    {
+        return $this->query('term', $params);
+    }
+
     /**
      * Write to the SOLR index.
      *