Add more unit tests

[Jorgelmh]

Description

Resolves #12

Add unit tests for the validation of the interface JSON, the modelDescription builder and the template data generated for the CPPFMU code.

How Has This Been Tested?

• Interface validation.
• FMU Template data.
• Model Description XML generator.

Screenshots (if appropriate)

Developer Checklist (before requesting PR review)

• My code follows the style guidelines of this project
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

I have:

• commented my code, particularly in hard-to-understand areas
• performed a self-review of my own code
• not committed unnecessary formatting changes, thereby occluding the actual changes (e.g. change of tab spacing, eol, etc.)
• made corresponding changes to the documentation
• added change to CHANGELOG.md
• added tests that prove my fix is effective or that my feature works (for core features)

Reviewer checklist

I have:

• have performed a review of the code
• tested that the software still works as expected
• [n/a] checked updates to documentation
• checked that the CHANGELOG is updated

[KristofferSkare]

Good work!

Just some minor comments and a suggestion on how the errors can be handled better with pytest.

[StephanieKemna]

Trying to test, but keep getting this error

which makes no sense to me. If this is building all in a venvwith local install of pythong, why would it care about some issues I have with system python install?

[StephanieKemna]

Here's the first part before the ros complaints

[StephanieKemna]

I think the pydantic dependency is not included - several files complain that pydantic could not be resolved.

[StephanieKemna]

I did a uv pip install lark, and with that I can run everything fine. So ignore above screenshots for this PR.

I but still think something is wrong with the uv venv though, since it should not need that. So doubting a bit the venv. I even manually activated the venv before trying to test this PR, but no difference.

[ClaasRostock]

@Jorgelmh
Regarding the matplotlib <-> Sphinx issue: I found the reason: In the python_project_termplate, I added a Sphinx plugin that needs matplotlib. That always worked, because all the projects I worked on until then had matplotlib as dependency anyhow.
mlfmu is the first project that doesn't, which is why it failed - and how we gain a learning :-)
So, the cleaner way for us to solve it is likely to remove the Sphinx plugin in question. Then we neither need matplotlib.
Let me check where exactly that change need to be.

[ClaasRostock]

@Jorgelmh
docs/source/conf.py -> line 40
"matplotlib.sphinxext.plot_directive",
-> remove that line.

Then also matplotlib can be removed as a dependency.

[KristofferSkare]

Just a small comment, but it is not important