[All] Define math::fabs function

src/math/fwd.hpp

src/math/quaternion.hpp

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

src/multibody/joint/joint-spherical.hpp

src/multibody/liegroup/special-euclidean.hpp

src/spatial/explog.hpp

src/spatial/inertia.hpp

unittest/finite-differences.cpp

unittest/joint-composite.cpp

unittest/liegroups.cpp