Add ability to update QuiltiX graph from in-memory MaterialX document

[kwokcb]

Issue

This allows for integrations which create an in-memory MaterialX doc which can be routed to QuiltiX to update it's graph.
Fixes #45

Changes

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.

[RichardFrangenberg]

Looks good, but then the mx_file_loaded signal makes more sense to be emitted from the load_graph_from_mx_file function. Then the load_graph_from_mx_doc wouldn't require a mx_file_path input attribute.

[kwokcb]

Thanks for catching this. I've remove the mx_file_path argument from load_graph_from_mx_doc and move the signal to load_graph_from_mx_file.