[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

[C++][Python][Major] Replaced the way collisions and visuals are stored in GeometryModel. Now it is two lists of GeometryObject (one for collisions, one for visuals) aggregating information instead of multiple vectors. Now handle multiple collisions/visuals for one link when parsing urdf file and keep track of the mesh absolute path. Changed binding accordingly.

Rework Joint Unit Tests and fixe bugs.

[C++][FixBug][Unittest] Fix bug in calc_aba (bug with boost:fusion preventing a proper acces to jmodel.axis) + reworked unittest for joint prismatic unaligned