[C++] Fix multiple bugs such as getting the correct name of geometryObjects, or initialize the bodyParents vector correctly, + binded some model datas

[C++][Python] Removed fixbodies in Model. Were a way to store visuals merged with Joints. Now this information is in GeometryModel (visuals) + updated python bindings

[C++] Make distinction between joint and body in Model API ( addJoint, appendBodyToJoint, addJointAndBody instead of addBody)

avoid the miss alignement on vector of Matrix6d

Correct travis + fix hpp-fcl issues

Fix call to old API

Added JointAccessor + its unittests.

 [C++][Unittest] Reworked JointAccessor Unittest. tests are now done on every type of joint of the boost variant Model

overload the new operator for the model and data classes

Add a cmake variable to choose if the pyhton wrapper will be compile or not

Handle the zero velocity case for spherical joint types

Completed the visitors on JointVariants for each method of the JointBase interface  and moved it to a different file.

Remove incorrect indents

Fix two bugs related to the API change

Python exposure tests

[C++][Python][Major] Replaced the way collisions and visuals are stored