Further improvements in Bullet MuJoCo MJCF import:

[MJCF import] improved humanoid.xml, inverted_pendulum.xml import (automatically add dummy-links to connect multiple joints between bodies)
[MJCF import] fix bug in quaternion conversion (w,x,y,z) -> (x,y,z,w)

examples/Importers/ImportURDFDemo/URDFImporterInterface.h

@@ -30,6 +30,7 @@ public:
 	/// optional method to provide the link color. return true if the color is available and copied into colorRGBA, return false otherwise
 	virtual bool getLinkColor(int linkIndex, btVector4& colorRGBA) const { return false;}
 
+	virtual int getCollisionGroupAndMask(int linkIndex, int& colGroup, int& colMask) const { return 0;}
 	///this API will likely change, don't override it!
 	virtual bool getLinkContactInfo(int linkIndex, URDFLinkContactInfo& contactInfo ) const  { return false;}