[C++] Reworked inclusion tree for joints, added visitors + minor fixes

src/algorithm/crba.hxx

src/multibody/joint/joint-basic-visitors.hpp

src/multibody/joint/joint-basic-visitors.hxx

src/multibody/joint/joint-composite.hpp

src/multibody/joint/joint-prismatic.hpp

src/multibody/joint/joint-variant.hpp

src/multibody/joint/joint.hpp

src/multibody/model.hpp

src/multibody/model.hxx

src/multibody/visitor.hpp

src/parsers/urdf/model.cpp

unittest/joint-composite.cpp

unittest/joint.cpp