add btFileUtils::toLower to convert a string (char*) to lower case

URDF import demo: add COLLADA .dae file support
add FiniteElementMethod demo, extracted from the OpenTissue library (under the zlib license)
don't crash if loading an invalid STL file
add comparison with Assimp for COLLADA file loading (disabled by default, to avoid library dependency)
Gwen: disable some flags that make the build incompatible

Demos3/FiniteElementMethod/OpenTissue/dynamics/fem/fem_uniform_density.h

@@ -0,0 +1,38 @@
+#ifndef OPENTISSUE_DYNAMICS_FEM_FEM_UNIFORM_DENSITY_H
+#define OPENTISSUE_DYNAMICS_FEM_FEM_UNIFORM_DENSITY_H
+//
+// OpenTissue Template Library
+// - A generic toolbox for physics-based modeling and simulation.
+// Copyright (C) 2008 Department of Computer Science, University of Copenhagen.
+//
+// OTTL is licensed under zlib: http://opensource.org/licenses/zlib-license.php
+//
+#include <OpenTissue/configuration.h>
+
+namespace OpenTissue
+{
+  namespace fem
+  {
+    namespace detail
+    {
+      /**
+      * Set Uniform Density.
+      *
+      * @param begin
+      * @param end
+      * @param density
+      */
+      template<typename real_type, typename tetrahedron_iterator>
+      inline void uniform_density(tetrahedron_iterator const & begin, tetrahedron_iterator const & end,real_type const & density)
+      {
+        assert(density>0 || !"uniform_density(): density must be positive");
+        for (tetrahedron_iterator T = begin;T!=end;++T)
+          T->m_density = density;
+      }
+
+    } // namespace detail
+  } // namespace fem
+} // namespace OpenTissue
+
+//OPENTISSUE_DYNAMICS_FEM_FEM_UNIFORM_DENSITY_H
+#endif