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/btMultiBodyConstraint.h

@@ -90,6 +90,10 @@ public:
 	void updateJacobianSizes();
 	void allocateJacobiansMultiDof();
 
+	//many constraints have setFrameInB/setPivotInB. Will use 'getConstraintType' later.
+	virtual void setFrameInB(const btMatrix3x3& frameInB) {}
+	virtual void setPivotInB(const btVector3& pivotInB){}
+
 	virtual void finalizeMultiDof()=0;
 
 	virtual int getIslandIdA() const =0;

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;
     }

src/BulletDynamics/Featherstone/btMultiBodyPoint2Point.h

@@ -53,11 +53,12 @@ public:
 		return m_pivotInB;
 	}
 
-	void setPivotInB(const btVector3& pivotInB)
+	virtual void setPivotInB(const btVector3& pivotInB)
 	{
 		m_pivotInB = pivotInB;
 	}
 
+
 	virtual void debugDraw(class btIDebugDraw* drawer);
 
 };

src/BulletDynamics/Featherstone/btMultiBodySliderConstraint.h

@@ -63,7 +63,7 @@ public:
 		return m_pivotInB;
 	}
 
-	void setPivotInB(const btVector3& pivotInB)
+	virtual void setPivotInB(const btVector3& pivotInB)
 	{
 		m_pivotInB = pivotInB;
 	}
@@ -83,7 +83,7 @@ public:
         return m_frameInB;
     }
     
-    void setFrameInB(const btMatrix3x3& frameInB)
+    virtual void setFrameInB(const btMatrix3x3& frameInB)
     {
         m_frameInB = frameInB;
     }