FIX: Normalize BIDS-URIs to subject-relative

[effigies]

Fixes #456.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 83.72%. Comparing base (18e5e95) to head (f9ac03c). Report is 9 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #458      +/-   ##
==========================================
+ Coverage   83.66%   83.72%   +0.05%     
==========================================
  Files          32       32              
  Lines        2816     2826      +10     
  Branches      377      380       +3     
==========================================
+ Hits         2356     2366      +10     
  Misses        390      390              
  Partials       70       70              

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[effigies]

@mgxd @oesteban A review would be appreciated.

[effigies]

Going to merge, and we can address any brittleness in a future PR.