Improvements to ReferentialSkeleton

[mxgrey]

This pull request is in response to issues #548 and #556. This fixes the issue where BodyNodes with zero-dof parent Joints will be left out of ReferentialSkeletons, and it allows users to add and remove BodyNodes and DegreesOfFreedom from Groups freely (i.e. without any coupling between BodyNodes and their parent DegreesOfFreedom).

However, in the current implementation there is still a 1-to-1 correspondence between BodyNodes and Joints, even though BodyNodes are decoupled from DegreesOfFreedom. What this means is that ReferentialSkeleton::getJoint(i) is always equivalent to ReferentialSkeleton::getBodyNode(i)->getParentJoint(). This is despite the fact that there is no coupling between ReferentialSkeleton::getBodyNode(~) and ReferentialSkeleton::getDof(~).

So I before this pull request can be considered finished, I'd like to pose the question: Should getBodyNode(~) be decoupled from getJoint(~) for ReferentialSkeletons, or is the current setup okay as it is?

I should also note that this pull request introduces a few changes to the behavior of the API of the Group class as a consequence of the decoupling. I hope that this does not result in any headaches for the existing users of Group.

[mxgrey]

The latest commit decouples the Joints in a Group from the BodyNodes in the Group as per the discussion in #548.

However, I should write some unit tests to test out these new features.

[mxgrey]

I've created unit tests for the new Group features. I believe this pull request is finished, unless someone spots an issue somewhere.

[jslee02]

👍 Looks good to me.