Added redundant Symbol.h header for backwards compatibility

gtsam/nonlinear/Symbol.h

@@ -0,0 +1,22 @@
+/* ----------------------------------------------------------------------------
+
+ * GTSAM Copyright 2010, Georgia Tech Research Corporation, 
+ * Atlanta, Georgia 30332-0415
+ * All Rights Reserved
+ * Authors: Frank Dellaert, et al. (see THANKS for the full author list)
+
+ * See LICENSE for the license information
+
+ * -------------------------------------------------------------------------- */
+
+/**
+ * @file Symbol.h
+ * @date Sept 1, 2013
+ * @author Frank Dellaert
+ * @brief Symbol.h was moved to inference directory, this header was retained for compatibility
+ */
+
+#pragma once
+
+#include <gtsam/inference/Symbol.h>
+