Better handle empty skeletons with assimp

[nmellado]

The fix introduced in #754 seems "correct", but we way need to refactor animation loading a bit.
I'm not an expert in assimp functionalities, maybe check some tutorial like http://ogldev.org/www/tutorial38/tutorial38.html

Originally posted by @dlyr in #754 (comment)

[MathiasPaulin]

The management of the skeleton loading in Radium correspond to what is described in the official assimp documentation http://assimp.sourceforge.net/lib_html/data.html.

Note that as assimp do not have the notion of skeleton (but only the notion of bones), and that inferring, in a general way, a skeleton from the node hierarchy is quite heuristic. In order to not miss needed nodes for the skeleton, the heuristic used in Radium is very conservative. That's why the old version defined empty skeleton that should be removed at the end.