Fix structural padding issue

[tknopp]

Fix the issue with structural padding when writing out ISMRMRD files.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (c4e3926) 86.68% compared to head (a7ffa11) 86.68%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #167 +/- ## ======================================= Coverage 86.68% 86.68% ======================================= Files 10 10 Lines 368 368 ======================================= Hits 319 319 Misses 49 49 ```

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