[constraint] makes BarycentricCenterEffector inherit from PositionEffector

[EulalieCoevoet]

This PR makes BarycentricCenterEffector inherit from PositionEffector which allows to:

• standardize the use of the data weight, directions, and useDirections
• adds the control of the orientation in case of Rigid3

Additionally:

• updates the example
• removes a test: it was testing the method initData() which I removed from the component because we use the initialization of PositionEffector now.

I'm not sure about the way I handled the deprecation warnings.

[EulalieCoevoet]

@alxbilger thanks for your review. I think we can merge now 😊

[alxbilger]

you added this right? I did not think about specialization for rigids when I did my suggestions. Have a look if you could use DataTypes::CPos to try to factorize the function

[EulalieCoevoet]

Well. I've made the changes. The only difference was the normalization of the orientation in the case of a Rigid.

So I've factorized the code by adding a setBarycenter() function which normalize the orientation in the case of a Rigid. It makes sens, even though it hides/disconnects the normalization after the accumulate. But I'm fine with it.

[EulalieCoevoet]

I'm sorry I really need this to be merged.
I think it's good to go but if you want other changes I can make another PR.