TST: Add test for DenseFieldTransform handling of OOB points

[oesteban]

Summary

Add a regression test ensuring dense displacement fields return original coordinates when queried outside their reference grid.

Closes: #202.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.24%. Comparing base (a03c51f) to head (31b6850).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #254   +/-   ##
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  Coverage   95.24%   95.24%           
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  Files          16       16           
  Lines        1976     1976           
  Branches      266      266           
=======================================
  Hits         1882     1882           
  Misses         75       75           
  Partials       19       19           

Flag	Coverage Δ
unittests	95.06% <ø> (ø)

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[oesteban]

Checked this test fails at 667344c~1 (that is, e792cef) and works with current master (after #188).