Blender Git Loki

Refactor Collada: Make BCAnimation class more self contained

Follwing funtionality was previously located in
BCAnimationSampler, but makes not much sense there:

* Moved the creation of the reference object into the Class constructor
* Moved the destruction of the reference to the Destructor

Furthermore:

* added get_reference() to better shield the rerference object from
unintentional access
* BCAnimationObjectMap now contains Pointers to BCAnimation objects
instead of containing full object instances. This avoids unnecessary
copying of the Animation datastructures when adding BCAnimation objects
to the BCAnimationObjectMap. Note: This forces us to delete the BCAnimation
objects in the BCAnimationSampler destructor.