fix some warnings, disable gimpact by default in world importer,

use DIRECTLY_UPDATE_VELOCITY_DURING_SOLVER_ITERATIONS by default for now,
until we find the issue with some failing test cases in btMultiBody
fix a crashing issue in MyMultiBodyCreator.cpp (uninitialized variable)
disable excessive debug printf in URDF2Bullet

examples/Importers/ImportURDFDemo/ImportURDFSetup.cpp

@@ -231,7 +231,7 @@ void ImportUrdfSetup::initPhysics()
 
 	if (loadOk)
 	{
-		printTree(u2b,u2b.getRootLinkIndex());
+		//printTree(u2b,u2b.getRootLinkIndex());
 
 		//u2b.printTree();