feat: add loop termination methods to VBMC

[Solosneros]

I implemented and tested three private methods to VBMC that originate from vbmc_termination.m.

• _is_finished(): Checks if loop termination conditions have been reached (max iterations, max fun evaluations, stability termination condition) and prevents early termination.
• _compute_reliability_index(): Computes the reliability index of VBMC.
• _is_gp_sampling_finished(): Checks if MCMC sampling of the Gaussian Process is finished.

Furthermore, I added pytest-mock to the dependencies. It helps to test methods independent of other methods or components. See test_vbmc_is_finished_stability(mocker) in pyvbmc/vbmc/test_vbmc_loop_termination.py for an example on how this improves the testability of the _is_finished method without having to care about the _compute_reliability_index() method.

[Solosneros]

I modified pyvbmc so that the initial iteration is 0 (in MATLAB it is 1). Furthermore, I added an interim solution to store the reliability index and stableflag. This will be improved later with the new iteration_history object.