Add support for IK in armatures

Kinematic chains can now be controlled via inverse kinematics
by placing/moving so-called IK targets that are tracked by your
armature.

A new service (Armature.place_IK_target) allows to instanteanously
place and orient an IK target, while the asynchronous service
Armature.move_IK_target allows to move at a given speed the target.

The list of IK targets available on a given armature can be obtain
with Armature.list_IK_targets.

To easily create the IK targets, the morse.builder.Armature offers the
create_IK_targets([joints]) that can be called in Builder scripts.

Note that internally, all rotations are handled as quaternions to
ease interpolation over time.

While here:
- removed the IK constraint in KUKA LWR Blend file
- added (from Python) an IK target to the KUKA LWR. Cf
  morse.builder.actuators.KukaLWR
  to see how this is done.
- updated testing/base/armature_testing
[doc] Updated RELEASE notes to mention IK targets
[doc] Mention IK-based (aka, cartesian) control of armatures