Various fixes regarding armatures, and rework of templates code

Templates were somehow buggy - you do not want several "ObjectType" nodes for a same type but different sub-types (e.g. NodeAttributes are used for lights, camera, limbnodes, etc.).

Also completed/fixed some armature stuff (mostly related to templates as well). Still not really working I think (FBX converted to collada gives correct armature and cube, but no skining, when imported back into Blender, and Houdini still crashes when trying to import those FBX from Blender).