create a map of the bones instead of searching for all the meshes

[NickHardeman]

Fixes load times mentioned in #8267 by caching the bone list instead of searching through all the meshes and all the bones for every isBone() function call.

[dimitre]

Lets merge this one

[NickHardeman]

This fix greatly reduces scene parsing by limiting recursion, though it's not really a bug, it's a vast improvement for loading large, complex scenes. Would be nice to get in for v0.12.1 :) What do you think @ofTheo

[ofTheo]

Was the issue in #8267 also related to textures or was this the main issue?

[NickHardeman]

There may still be a texture issue, but this was the main slow down ( 30 seconds ), now resulting in load times comparable to the older ofxAssimpModelLoader ~13 seconds.