FIX: Add per-volume transforms as single transform in chain

[effigies]

I believe 9772710 caused TransforrmChain to start treating LinearTransformMappings as Iterables to decompose. Thus, I was getting transform chains 303 transforms long instead of 4 long.

Here is a regression test that fails before the patch and passes after the patch.

I believe this resolves the issues in fMRIPrep/fMRIPost.

cc @tsalo

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.76%. Comparing base (e9b46cf) to head (7bc22c5).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #226   +/-   ##
=======================================
  Coverage   94.76%   94.76%           
=======================================
  Files          15       15           
  Lines        1756     1758    +2     
  Branches      328      329    +1     
=======================================
+ Hits         1664     1666    +2     
  Misses         76       76           
  Partials       16       16           

Flag	Coverage Δ
unittests	94.65% <100.00%> (-0.11%)	⬇️

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