FIX: Deltas of displacements fields were overwritten

[oesteban]

When calculating the field itself. This prevented round-trip tests writing out and then loading transforms.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.09%. Comparing base (2f66df4) to head (3e09d11).
Report is 30 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #251      +/-   ##
==========================================
+ Coverage   94.91%   95.09%   +0.18%     
==========================================
  Files          16       16              
  Lines        1908     1918      +10     
  Branches      206      251      +45     
==========================================
+ Hits         1811     1824      +13     
+ Misses         77       75       -2     
+ Partials       20       19       -1     

Flag	Coverage Δ
unittests	94.91% <0.00%> (ø)

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