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]

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 ⚠️

Additional details and impacted files

@@            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...