FIX: Use subject root to resolve IntendedFor paths

[effigies]

It looks like we'd resolved this once with intended_root, but the root is always the subject folder.

Closes #227.

[codecov-commenter]

Codecov Report

Merging #228 (ab24b17) into master (592cacc) will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #228      +/-   ##
==========================================
+ Coverage   93.53%   93.66%   +0.12%     
==========================================
  Files          24       24              
  Lines        1671     1673       +2     
  Branches      192      191       -1     
==========================================
+ Hits         1563     1567       +4     
+ Misses         82       81       -1     
+ Partials       26       25       -1     

Flag	Coverage Δ
travis	88.94% <100.00%> (?)
unittests	93.52% <100.00%> (-0.02%)	⬇️

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Impacted Files	Coverage Δ
sdcflows/utils/wrangler.py	100.00% <100.00%> (ø)
sdcflows/utils/bimap.py	100.00% <0.00%> (ø)
sdcflows/fieldmaps.py	99.40% <0.00%> (+0.01%)	⬆️
sdcflows/transform.py	97.61% <0.00%> (+0.02%)	⬆️
sdcflows/interfaces/bspline.py	93.71% <0.00%> (+1.04%)	⬆️

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[effigies]

This test reproduced #227. Used ellipses because I had to comment out the datalad datasets to test. I assume it's okay to leave them there.