cleanup & reorg: 

- add convert folder for all OpenCOLLADA-> GLTF model conversion
- moved all animations related conversion code from writer to convert folder
- add math helpers (and cleared main writer from math related code).
- rename functions for consistent naming across the project
- created GLTF-OpenCOLLADA.h  include to centralize all OpenCOLLADA includes
move mesh conversion out from writer to convert folder.