[FIX] force_fmapless on top of self-correcting pepolar bold

[bpinsard]

When force_fmapless is used on top of self-correcting pepolar bold, the fmapless estimator will get it's name from B0FieldIdentifier, creating a duplicate.
For now I just created a test to replicate that behavior and triggering the error that this PR will try to fix.

[bpinsard]

Also, it seems there was no test for force_fmapless=True.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.70%. Comparing base (656a51d) to head (dc6d2f7).

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HEAD has 8 uploads less than BASE

Flag	BASE (656a51d)	HEAD (dc6d2f7)
	12	4

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #465      +/-   ##
==========================================
- Coverage   83.75%   74.70%   -9.05%     
==========================================
  Files          32       32              
  Lines        2831     2831              
  Branches      381      284      -97     
==========================================
- Hits         2371     2115     -256     
- Misses        390      653     +263     
+ Partials       70       63       -7     

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