Make it possible to enable/disable the EVC

[bouthilx]

The experiment version control (EVC) is a good fail-safe to protect users from unintended modifications in the HPO pipeline that may affect the results. The problem is that it can get in the way if users want to modify their pipeline without branching the experiments, making the experience painful. Branching is also a confusing feature for many new comers which in the end causes more issues than it solves.

The EVC should be disabled by default, with an option to enable it. When EVC is disabled, we should still verify overwriting of experiment configurations and warn about it.

@mgermain