Moved joint::nv as a function.

src/algorithm/crba.hpp

src/multibody/constraint.hpp

src/multibody/joint/joint-base.hpp

src/multibody/joint/joint-free-flyer.hpp

src/multibody/joint/joint-generic.hpp

src/multibody/joint/joint-revolute.hpp

src/multibody/joint/joint-variant.hpp

src/multibody/model.hpp

unittest/crba.cpp