RB constraint solver traceable by CppAD, inclined plane demo using SDF rendering + rigid body

[eric-heiden]

• Improve gradient stability in norm/normalize, quat_to_matrix functions in EigenAlgebra: we should always use Algebra::normalize(Vector3) instead of manually writing Vector3 / norm(Vector3)
• RB constraint solver traceable by CppAD
• implement Box geometry + contact handling (8 spheres in corner points for plane contact) and URDF parsing
• add inclined plane demo inclined_plane_gui where a geometry is created and simulated via rigid bodies, multi bodies (floating-/fixed-base); visualization via SDF meshing code
• Small fixes (e.g. World::clear() now doesn't segfault anymore in some cases)
• some code formatting via clang-format with Google style (added .clang-format to root)

[erwincoumans]

Thanks, I reviewed the changes, LGTM.