Fix 4D I/O incompatibility with ITK

[fepegar]

Fixes #285.

[codecov]

Codecov Report

Merging #286 into master will decrease coverage by 0.17%.
The diff coverage is 86.79%.

@@            Coverage Diff             @@
##           master     #286      +/-   ##
==========================================
- Coverage   92.98%   92.81%   -0.18%     
==========================================
  Files         108      108              
  Lines        5063     5035      -28     
==========================================
- Hits         4708     4673      -35     
- Misses        355      362       +7     

Impacted Files	Coverage Δ
...ms/preprocessing/test_histogram_standardization.py	100.00% <ø> (ø)
tests/utils.py	94.87% <66.66%> (-2.50%)	⬇️
torchio/utils.py	83.24% <75.00%> (-5.76%)	⬇️
torchio/data/image.py	90.16% <85.71%> (+<0.01%)	⬆️
torchio/data/io.py	96.24% <89.47%> (+2.25%)	⬆️
tests/data/test_image.py	100.00% <100.00%> (ø)
tests/data/test_io.py	100.00% <100.00%> (+1.02%)	⬆️
...eprocessing/intensity/histogram_standardization.py	94.26% <100.00%> (ø)
... and 2 more

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