Simply support 3MF Production Extension.

[cmguo]

This patch adds simple support for [3MF Production Extension]https://github.com/3MFConsortium/spec_production/blob/master/3MF%20Production%20Extension.md).

I use an implicit method to extend the function instead of modifying the py interface.
This method prepends the "path" to the xml content and passes them together to ThreeMFParser::parse.

The parse method works with a single scene until it reaches the root model. The single scene is a static C++ instance, because I can't add members to the class ThreeMFParser , that would break the consistency with the pySavitar library.

[cmguo]

The unit-test fail is not relate to source code.

Notice: A new release of pip is available: 23.0.1 -> 23.1.2
Notice: To update, run: pip install --upgrade pip
usage: conan profile [-h] [-v [V]] {detect,list,path,show} ...
conan profile: error: argument subcommand: invalid choice: 'new' (choose from 'detect', 'list', 'path', 'show')
ERROR: Exiting with code: 2
Error: Process completed with exit code 2.
0s

[rburema]

A unit-test fix was just committed to main, should work again.

We're (or rather, my colleagues) are working to clean up our PR backlog, so hopefully the wait (at least for go/no-go) won't be that much longer.