FIX: Restore access to private attr Nifti1Extension._content

[effigies]

gh-1336 reused the private attribute ext._content to exclusively refer to the bytes representation of the extension contents. This neglected that subclasses might depend on this implementation detail.

Let's be nice to people and rename the attribute to _raw and provide a _content property that calls self.get_content().

Also adds a test to ensure that multiple accesses continue to work as expected.

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 2 lines in your changes missing coverage. Please review.

Project coverage is 86.45%. Comparing base (1712cb0) to head (e97f572).
Report is 3 commits behind head on maint/5.3.x.

Files with missing lines	Patch %	Lines
nibabel/tests/test_nifti1.py	88.23%	1 Missing and 1 partial ⚠️

❗ There is a different number of reports uploaded between BASE (1712cb0) and HEAD (e97f572). Click for more details.

HEAD has 27 uploads less than BASE

Flag	BASE (1712cb0)	HEAD (e97f572)
	29	2

Additional details and impacted files

@@               Coverage Diff               @@
##           maint/5.3.x    #1378      +/-   ##
===============================================
- Coverage        95.37%   86.45%   -8.93%     
===============================================
  Files              207      207              
  Lines            29652    29672      +20     
  Branches          3017     3018       +1     
===============================================
- Hits             28282    25654    -2628     
- Misses             933     3587    +2654     
+ Partials           437      431       -6     

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[effigies]

@bpinsard Thanks for checking that this works as intended. Would you have time for a code review as well?

[bpinsard]

Not 100% familiar with the intricacies of nifti extension but these changes look good to me. Thanks!

[effigies]

Thanks!