[C++] Move operator== to the JointModelBase class

src/multibody/joint/joint-base.hpp

src/multibody/joint/joint-dense.hpp

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

src/multibody/joint/joint-planar.hpp

src/multibody/joint/joint-prismatic-unaligned.hpp

src/multibody/joint/joint-prismatic.hpp

src/multibody/joint/joint-revolute-unaligned.hpp

src/multibody/joint/joint-revolute-unbounded.hpp

src/multibody/joint/joint-revolute.hpp

src/multibody/joint/joint-spherical-ZYX.hpp

src/multibody/joint/joint-spherical.hpp

src/multibody/joint/joint-translation.hpp

src/multibody/joint/joint.hpp