Allowing OOs to use precomputed RGE matrix with fixed scale.

[arossia94]

So far, if one inputted a pre-computed RGE matrix with a fixed observable scale, the Optimal Observable module would ignore it and recompute the RGE with the observable scale that corresponds to each optimal observable.

This would cause incorrect results in the following situations:

• When one wants to compare the impact of a fixed observable-scale for the RGE running, since the OOs would be computed as if it was the dynamical mode.
• When one wants to use an RGE matrix computed outside SMEFiT.

This PR addresses both issues.

[arossia94]

The tests seem to fail because they run on the latest release of smefit (3.1.1, from May 2025), which hadn't implemented the function load_precomputed_rge_matrix within the RGE module. However, this interface runs satisfactorily with the latest smefit version on the main branch of the github repo.

[arossia94]

Not prioritary for the ESPPU, but would affect if we do UV fits or mass scans with fixed observable scale.