Allow for load in MaterialX documents from memory

[kwokcb]

Proposal

This is a proposal to split up the current load from filename into 2 parts:

1. Load from in memory document
2. Load from filename which calls 1.

Requirement

This allows for an integration which loads in MaterialX documents on it's own to route this to QuiltIX. In my use case this is to load glTF files into MTLX documents and then updateing the QuiltiX graph. (See https://github.com/kwokcb/QuiltiX/blob/gltf_plugin/plugins/pluginQuiltX.py#L36 ).

Changes

The changes are relatively minor to split out the file read part of load_graph_from_mx_file into a load_graph_from_mx_doc (load from in memory document). The file handling part would stay in load_graph_from_mx_file which would call load_graph_from_mx_doc.