add pybullet.changeConstraint / b3InitChangeUserConstraintCommand/ b3InitChangeUserConstraintSetPivotInB /b3InitChangeUserConstraintSetFrameInB command, to change an existing user constraint.

add constraint.py example.
allow pybullet.createConstraint to create user constraint without a child body ('fixed' to the world)

src/BulletDynamics/Featherstone/btMultiBodyFixedConstraint.h

@@ -62,7 +62,7 @@ public:
 		return m_pivotInB;
 	}
 
-	void setPivotInB(const btVector3& pivotInB)
+	virtual void setPivotInB(const btVector3& pivotInB)
 	{
 		m_pivotInB = pivotInB;
 	}
@@ -82,7 +82,7 @@ public:
         return m_frameInB;
     }
     
-    void setFrameInB(const btMatrix3x3& frameInB)
+    virtual void setFrameInB(const btMatrix3x3& frameInB)
     {
         m_frameInB = frameInB;
     }