Materialxgltf

What's Changed
* Default inputs, physical materials, command line access. by kwokcb in https://github.com/kwokcb/materialxgltf/pull/4
* Error string fix by kwokcb in https://github.com/kwokcb/materialxgltf/pull/5
* Fix `attenuation_color` read from glTF. by kwokcb in https://github.com/kwokcb/materialxgltf/pull/7
* Support adding extract nodes on gltf to mtlx by kwokcb in https://github.com/kwokcb/materialxgltf/pull/8

**Full Changelog**: https://github.com/kwokcb/materialxgltf/compare/v0.0.3...v0.0.4

Core Updates
- Add package version number
- Updates to Jupyter book, gallery and readme
- Fix baking calls to use proper search path option
- Find absolute paths to resources for data packing to GLB
- Handle no material case (mtlx)
- Add material if none (gltf)
- Find absolute paths to resources for data packing (gltb)
- Add GLTF material if none found in GLTF
- Gracefully handle when there are no materials found in MTLX

Test Updates
- Add GLTF->MTLX test suite folder.
- Add in glTF -> MTLX render examples

Example Updates
- Add Adobe Substance 3D example with different "shader ball"
- Add Adobe Substance 3D example with different "shader ball" geometries
* Add in "Physically Based" Examples.

Initial Release

Contains Python Package with the following components:
- `core` : classes to convert to and from glTF / MaterialX
- `util` : classes for support functions such as MaterialX I/O
- top level convenience functions and commands
- doxygen documentation
- samples from MaterialX examples library.