create testing data for released weldx versions

[marscher]

To ensure backward-compatibility with previously created files, we should create at one weldx file for each released version.
It would be preferable having a separate repository for these, which could be included as submodule in this repository. This would enable for easier CI operations.

We can use the single_pass_weld schema here, or more fine grained schemas. I guess the more fine grained they are, the easier it would be to spot broken things. However it is not expected for things to break often, so the big schema would be preferable then.

[CagtayFabry]

another option would be to run the pytest asdf plugin on old schema files as they include (a lot of) example code with correctly tagged (legacy) versions
I will test if this works as expected (but from now on all legacy schemas will be included in the weldx main repository anyway so this testing method would come for free if feasible 👍 )

[marscher]

Sounds like a feasible solution to me.