[WIP] Recompress files where dcm2niix failed

[octomike]

As threatened in #802 an alternative approach.

• try to open and read the nifit to verify it wasn't truncated
• recompress with gzip
• fail if we couldn't - as opposed to just emit invalid file names (at least fmriprep fails to handle nii.gz that are not gz at all)

I decided to leverage the fact that heudiconv blindly moves all converted output files to nii.gz, even if they are just nii by renaming the successfully re-compressed image to .nii. This feels ugly, but results in the shortest/least-invasive PR.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 28.00000% with 18 lines in your changes missing coverage. Please review.

Project coverage is 82.12%. Comparing base (900ccdc) to head (a72d5ec).

Files with missing lines	Patch %	Lines
heudiconv/convert.py	28.00%	18 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #803 +/- ## ========================================== - Coverage 82.48% 82.12% -0.36% ========================================== Files 42 42 Lines 4323 4348 +25 ========================================== + Hits 3566 3571 +5 - Misses 757 777 +20 ```

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