[builder] Extended significantly the Armature builder helper

In particular, it is now (rightfully) a subclass of actuator, and it supports
adding IK targets to drive specific bones.
 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.

 Known bug:
 - setting a rotation 'over time' (with move_IK_target) is currently
   disabled because I could not figure out a good way to interpolate the
   angles over time. Help would be welcome!

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