Add new possiblities for providing paramters.

[pat-schmitt]

This PR introduces two new ways to define and provide parameters for the model (in addition to the current default of using cfg.PARAMS):

1. Minimal params.cfg File: You can now pass a minimal params.cfg file to cfg.initialize. If some parameters are missing in this file, the defaults from the main params.cfg (available here) will be used automatically.
2. New run_settings.yml File, which could contain:

• Almost all parameters from params.cfg
• Values of observations used during calibration or initialization
• If any parameter is not included in run_settings.yml, it falls back to the current defaults.

These additions required significant changes in sensitive parts of the model, particularly for the second point. However, tests appear to be passing. I have also included some tests for run_settings.yml. Not every aspect is tested yet, as adding more tests would overly expand the test suite.

Points for Discussion

• Each function currently opens the run_settings.yml file independently, instead of opening it once at the start of a workflow and passing it around. While this approach may slightly reduce performance (dynamic runs are not effected by this), it maintains the flexibility needed for setting up custom workflows.

• The new run_settings.yml works similarly to the existing mb_calib logic but supports calibration for only a single flowline. For now, I’ve kept mb_calib for backward compatibility and enforce a selection between the two possibilities to avoid potential confusions. However, we could consider removing mb_calib in the future.

• When using run_settings.yml, all diagnostics are saved both to gdir.add_diagnostics and the run_settings.yml file. Maybe one or the other is enough?

• Tests added/passed

• Fully documented

• Entry in whats-new.rst

[pep8speaks]

Hello @pat-schmitt! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file oggm/core/massbalance.py:

Line 1899:22: E241 multiple spaces after ','