FIX: Revision of the B-Spline fitting code

[oesteban]

Several issues are addressed:

• Fit intercept: instead of removing the mean/median/mode before fitting, let the model account for that by fitting an intercept.
• Lower regularization alpha: it seems the regularization can be much lower when fitting only one level of b-splines
• Update the workflow so only one level of b-splines is fit (multi-level fitting is not working, I believe)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 85.71429% with 2 lines in your changes missing coverage. Please review.

Project coverage is 76.33%. Comparing base (bb89bdc) to head (1883c15).

Files	Patch %	Lines
sdcflows/workflows/fit/fieldmap.py	33.33%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #453 +/- ## ========================================== + Coverage 76.21% 76.33% +0.11% ========================================== Files 32 32 Lines 2809 2814 +5 Branches 376 376 ========================================== + Hits 2141 2148 +7 + Misses 600 598 -2 Partials 68 68 ```

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[oesteban]

This resolves Sunjae's dataset issue and brings all test back to green. Merging...