Materialxjson

- Release to match with core MaterialX 1.39 release.
- No core logic changes from 1.38.x. Only rerun Jupyter notebook and scripts to examples.
- Mark as a proper release vs pre-release.

This branch exists to for compatibility with 1.38.9.
Mostly it is used to build the examples and docs and to generate a 1.38.9 PyPi package from.

- Pre-release of JSON I/O
- First release added to `PyPi`
- Includes:
- [x] 1:1 Lossless read / write to JSON format
- [x] JSON Read Options:
- [x] Upgrade to latest MTLX version
- [x] JSON Write Options
- [x] Element predicate for filtering library elements, types etc
- [x] Formatting options
- [x] Mimetype write and check
- [x] Command line utilities for read (mtlx2json) / write (json2mtlx)
- [x] Python / Doxygen docs
- [x] Sample in Jupyter notebook