ENH: Update visual report specification with fieldmaps

[oesteban]

Brings the specification from dMRIPrep, covering the new SDCFlows capabilities.

[codecov-commenter]

Codecov Report

Merging #634 (939a5ac) into master (5792f74) will decrease coverage by 0.72%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #634      +/-   ##
==========================================
- Coverage   48.57%   47.84%   -0.73%     
==========================================
  Files          45       45              
  Lines        5501     5497       -4     
  Branches      790      786       -4     
==========================================
- Hits         2672     2630      -42     
- Misses       2729     2773      +44     
+ Partials      100       94       -6     

Flag	Coverage Δ
masks	?
reportlettests	∅ <ø> (∅)
unittests	47.84% <ø> (ø)

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Impacted Files	Coverage Δ
niworkflows/func/util.py	17.97% <0.00%> (-31.47%)	⬇️
niworkflows/interfaces/fixes.py	50.00% <0.00%> (-5.56%)	⬇️
niworkflows/interfaces/header.py	51.70% <0.00%> (-4.28%)	⬇️
niworkflows/interfaces/reportlets/base.py	42.85% <0.00%> (-1.59%)	⬇️
niworkflows/viz/utils.py	8.84% <0.00%> (+0.02%)	⬆️
niworkflows/interfaces/norm.py	25.86% <0.00%> (+0.14%)	⬆️
niworkflows/interfaces/images.py	66.88% <0.00%> (+0.21%)	⬆️
niworkflows/utils/bids.py	75.00% <0.00%> (+0.74%)	⬆️

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