Fix planar joints integration

[gergondet]

Following #33, integration for planar joints needs to be looked at

[gergondet]

So, I wanted to assign this to @aescande but I cannot for some reasons 😿

[haudren]

Maybe because he is not a member of jrl-umi3218?

[gergondet]

He is but he is a private member so that may be the issue...

[aescande]

shhhh, don't tell everyone ;)