Make transfer_model more user-friendly

[gngdb]

I added some example code, sample data and steps for how to use the transfer_model utility. Also, I updated the install instructions based on my experience, which should keep working for a bit longer.

Also, I cleaned up joint_names.py and added a class to make it easier to convert between SMPL and SMPL-H joint names, which is faster and more convenient than running the transfer_model.py utility when converting between those two body models. Although, it may not be preferred because I think two hand joints get dropped in the conversion instead of getting matched to the set of hand joints in SMPL-H, is that right?

Body class usage is based on scipy.spatial.transform.Rotation. If we have some body pose data x in SMPL format (rotational formalism doesn't matter) that is an array of shape (number of joints, number of rotational dimensions), convert it to SMPL-H array y: y = Body.from_smpl(x).as_smplh().

This pull request would address #75, #82 and other issues.

[vchoutas]

@gngdb can you update the description for the Trust NCG optimizer? I fixed the packaging in that repo, so the rollback is no longer needed. The two hand joints that are dropped don't have a correspondence in SMPL+H, so no harm done. Thank you for the pull request!