Setup data that is used to create the representation of a single limb. A limb is an array of contiguous bones (e.g. left arm, or the spine etc). We can define it has the set of bones that are children of a start bone (plus the start bone itself), plus optionally the parent of that start bone (this is useful when defining the spine, since you will want to include the pelvis, but you don't to include all the children of the pelvis since that would include the legs), but excluding any bones that are already part of another limb. This implies limbs should be constructed in order from leaf to root.