Refactor folder (#10475)

Move many third-party components' source codes from the src folder to a new folder called deps_src. The goal is to make the code structure clearer and easier to navigate.

deps_src/libigl/igl/PI.h

@@ -0,0 +1,19 @@
+// This file is part of libigl, a simple c++ geometry processing library.
+// 
+// Copyright (C) 2013 Alec Jacobson <alecjacobson@gmail.com>
+// 
+// This Source Code Form is subject to the terms of the Mozilla Public License 
+// v. 2.0. If a copy of the MPL was not distributed with this file, You can 
+// obtain one at http://mozilla.org/MPL/2.0/.
+#ifndef IGL_PI_H
+#define IGL_PI_H
+namespace igl
+{
+  // Use standard mathematical constants' M_PI if available
+#ifdef M_PI
+  const double PI = M_PI;
+#else
+  const double PI = 3.1415926535897932384626433832795;
+#endif
+}
+#endif