Remove a bunch more indirection.

Mainly affects the inertia properties.
Merge branch 'indigo-devel' of https://github.com/g/grizzly into remove-properties
A few more tidy-ups here.

- More joints to after their respective child links.
- Group wheels together at the end.
- Re-add IMU orientation.
Merge pull request #12 from g/remove-properties

Remove a bunch more indirection.