Implement InMemoryFile for memory caching for fileIO plugin.

Support SDF loading through fileIO plugin.
Replace strcspn by C code (not crossplatform)
Add flag for loadURDF to use color from MTL file (instead from URDF link material)
pybullet.URDF_USE_MATERIAL_COLORS_FROM_MTL and pybullet.URDF_USE_MATERIAL_TRANSPARANCY_FROM_MTL

examples/Importers/ImportURDFDemo/URDF2Bullet.h

@@ -32,6 +32,8 @@ enum ConvertURDFFlags
 	CUF_INITIALIZE_SAT_FEATURES = 4096,
 	CUF_USE_SELF_COLLISION_INCLUDE_PARENT = 8192,
 	CUF_PARSE_SENSORS = 16384,
+	CUF_USE_MATERIAL_COLORS_FROM_MTL = 32768,
+	CUF_USE_MATERIAL_TRANSPARANCY_FROM_MTL = 64738,
 };
 
 struct UrdfVisualShapeCache