copy header when creating new nibabel objects

[cakester]

Checklist

GitHub

• I've given this PR a concise, self-descriptive, and meaningful title
• I've linked relevant issues in the PR body
• I've applied the relevant labels to this PR
• I've assigned a reviewer

PR contents

• I've consulted ivadomed's internal developer documentation to ensure my contribution is in line with any relevant design decisions
• I've added relevant tests for my contribution
• I've updated the relevant documentation for my changes, including argparse descriptions, docstrings, and ReadTheDocs tutorial pages

Description

Linked issues

ivadomed#715

[coveralls]

Pull Request Test Coverage Report for Build 730006326

• 2 of 8 (25.0%) changed or added relevant lines in 5 files are covered.
• 4 unchanged lines in 3 files lost coverage.
• Overall coverage remained the same at 75.631%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
ivadomed/evaluation.py	0	1	0.0%
ivadomed/visualize.py	0	2	0.0%
ivadomed/uncertainty.py	0	3	0.0%

Files with Coverage Reduction	New Missed Lines	%
ivadomed/evaluation.py	1	11.71%
ivadomed/visualize.py	1	37.76%
ivadomed/uncertainty.py	2	11.83%

Totals
Change from base Build 686790450:	0.0%
Covered Lines:	5456
Relevant Lines:	7214

---

💛 - Coveralls

[cakester]

all changed except for
https://github.com/ivadomed/ivadomed/blob/fb3c913ddc3fb1942884bdbb16a7796e94d20192/ivadomed/loader/adaptative.py#L753