Implement proposal from issue #103 for simplifying the pointer structure between joints and body nodes.

The SDF and Skel parsers, the apps and unittests are not adapted to the changes yet and have been  excluded temporarily from compilation.

The following variables and functions have been removed:

- BodyNode::mChildJoints and its accessor functions
- Joint::mParentBodyNode and its accessor functions
- Joint::mChildBodyNode and its accessor functions
- Joint::updateKinematics() (updateTransform(), updateVelocity() and updateAcceleration() have been made public instead)
- Skeleton::mJoints
- Skeleton::addJoint()
- Skeleton::getNumJoints()
- RevoluteJoint::getWorldAxis() (similarly for other joint types)
- RevoluteJoint::getWorldOrigin() (similarly for other joint types)
Rename files
Adapt SkelParser, SdfParser, unittest, and apps to issue #103
Remove BodyNode::setParentBodyNode()
Merge remote-tracking branch 'remotes/origin/pointers'