FIX: Remove "densification" of B-Spline transforms in apply()

[oesteban]

For some reason, apply() always converted B-Spline displacements into a displacements dense-field.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.68%. Comparing base (20e2c25) to head (cdcc9bb).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #280      +/-   ##
==========================================
+ Coverage   95.59%   95.68%   +0.09%     
==========================================
  Files          16       16              
  Lines        1996     1994       -2     
  Branches      268      267       -1     
==========================================
  Hits         1908     1908              
+ Misses         65       64       -1     
+ Partials       23       22       -1     

Flag	Coverage Δ
unittests	95.66% <100.00%> (+0.09%)	⬆️

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