feat: basic FMI3 CS doStep implementation

[PeterMeisrimelModelon]

Not really re-viewable commit-by-commit.

Test re-structuring:

1. Moved the relevant new fixtures for FMI3 CS testing to conftest.py and the relevant helper functions to the new utils.py.

Comments for the actual changes:

1. My take of the FMI3 spec is that "terminate" is not formally included in the "earlyReturn", which I base on the https://fmi-standard.org/docs/3.0.2/#_basic_co_simulation in the specification. I.e., FMUs are allowed to return with terminateSimulation = True, even if instantiated with allowEarlyReturn=False
2. terminate now being part of the do_step signature is a breaking change, so we need a new mechanism to handle this. I think the 3 possible ways are a class attribute (the current variant), a helper function that fulfills the same purpose as the attribute, or make do_step have multiple returns. Multiple returns might be most aligned with the underlying C-API behavior, but least consistent with FMI2, but then again, this is a breaking change. Looking for input/suggestions here.

[efredriksson-modelon]

👍

[efredriksson-modelon]

🍉

(but please consider tweaking the variable names 😄)