Add unit testing framework and tests for existing code (and assorted fixes and breaking API changes)

[Sophon96]

Using catch2 and integration with meson's testing harness (so tests can be run with meson test).

[Sophon96]

Incomplete, blocked for test requirements from @adamhutchings for the matrix and vector classes.

[adamhutchings]

Are we merging this after #25?

[Sophon96]

Are we merging this after #25?

No, I need to write tests for the existing code as well (though this is blocked by #25 as well).

[adamhutchings]

Is development on this able to proceed or are we still waiting on something else?

[Sophon96]

Is development on this able to proceed or are we still waiting on something else?

Waiting on nothing, just time and writing tests. I think it would be interesting if we want to do test-driven development or behavior-driven development for v1 (i.e. draft the API, write all the tests, and once all tests pass v1 is complete).

[adamhutchings]

I think that is a worthwhile discussion to have, but if we have the basis of testing set up, we can probably close this PR and add more tests as they come (or however else we agree to do it).

[Sophon96]

Blocked #38

[Sophon96]

TODO before this can be merged

• Update docs (just README.md for now)
• Tests for the rest of the code (Model, though not the entirety because of the lack of actual implementation; getters and setters)
• Add more getters (Matrix needs some, Model too)
• Review refactored code and breaking API changes introduced in this PR.

[Sophon96]

C API is not tested.

[Sophon96]

@adamhutchings @JakeRoggenbuck Ready to review

[adamhutchings]

Much appreciated!

[adamhutchings]

I don't think this is UB because we can just arbitrarily choose a value for the derivative. Like you chose to do, probably better not to test this because the behavior of the derivative at the discontinuity doesn't matter.