Allows skeleton to have multiple parent-less body nodes

* When a skeleton has multiple parent-less body nodes, then the skeleton computes forward dynamics for only the first added parent-less body node (as root body node) and its child body nodes. For example, a skeleton has two body nodes without any joint that connects them updates only (added first to the skeleton) one body node. Gazebo allows these case for model. This change doesn't have any negative effect on the dynamics algorithms of DART.
* Target DART version is 4.0.1