Animation loader improvements

First, get rid of the unhelpful "your scene has too many animations" warning by going through all animations in the assimp scene when splitting (everything is treated as being part of the same timeline).

Secondly, give an error if two animations are directly controlling the same node. That sort of advanced scenario is not currently supported.

If you want to really test, the sample models repository has singleAnim and multiAnim test models.