add URDF importer

[baggepinnen]

WIP

TODO

• [x] Transform inertia matrices
  • The inertial spatial transform can be provided to the r_cm keyword, and the rotation can be done with resolve_dyade
• [x] Handle joint transformations, origin in spec
• [ ] handle friction (separate from damping)
• [x] The handling of r in BodyShape is probably not correct, joints store " This is the transform from the parent link to the child link. The joint is located at the origin of the child link, as shown in the figure above.", i.e., the r of a link is stored in the following joint :O
• [x] docs
• [x] extension
• [x] Handle gravity
• [ ] handle all geometry types
  • [x] cylinder
  • [x] box
  • [x] sphere
  • [ ] mesh