Add support for multiple animations per file

[sbarthelemy]

Currently all the animation curves are stored directly under the animation_library element.

<animation_library>
    <animation id="Arm-matrix">...</animation>
    <animation id="Forearm-matrix">...</animation>
</animation_library>

We could use a hierarchy of animations:

<animation_library>
    <animation id="simu0">
        <animation id="simu0-Arm-matrix">...</animation>
        <animation id="simu0-Forearm-matrix">...</animation>
    </animation>
    <animation id="simu1">
        <animation id="simu1-Arm-matrix">...</animation>
        <animation id="simu1-Forearm-matrix">...</animation>
    </animation>
</animation_library>

We should then add a cli option to get the animation name/id (here simu0, simu1).

What's the point?

• [+] support several animations per file
• [-] We'll need to use complex id such as #anim0-Arm-matrix-interpolation.
  Maybe a better solution is continue with an animation per file, but keep the scene in a
  distinct file. The animation file name would then serve as a name space,
  (as in anim0.dae#Arm-matrix-interpolation)

This is an example of a collada file with a hierarchy of (two) animations :
http://gist.github.com/610049#file_animation_hierarchy.dae

It passes coherencytest. However osganimationviewer only sees one animation (the first). In order to get multiple animation per file support in OSG, we should use animation_clips.