Automatic infer inertia from geometry

[imoneoi]

Automatic infer inertia from collider geometry like MuJoCo, replacing the arbitrarily set default values, as an identity matrix is often too large.

If mass / inertia is not set, it will be inferred from geometry. (assuming 1000 kg / m^3 density)

See #240

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[erikfrey]

Thank you for this PR!

Brax will soon support loading directly from mujoco xml files, and as part of that change we'll automatically calculate inertia just like mujoco does. I'm going to close this PR in favor of that upcoming change, which we'll be pushing out a preview of in the coming days.