New variable: Containing parent record

* php/HAB/Pica/Record/Record.php ($_parent): New variable. Containing
parent record.

src/php/HAB/Pica/Record/Record.php

@@ -87,6 +87,13 @@ abstract class Record {
    */
   protected $_fields = array();
 
+  /**
+   * The containing parent record, if any.
+   *
+   * @var \HAB\Pica\Record
+   */
+  protected $_parent;
+
   /**
    * Constructor.
    *