type: Relax Nifti1Image to SpatialImage

[effigies]

@jhlegarreta I meant to push this onto #150 after your comment but didn't get around to committing yesterday.

[codecov]

Codecov Report

Attention: Patch coverage is 54.54545% with 5 lines in your changes missing coverage. Please review.

Project coverage is 65.21%. Comparing base (d88ca78) to head (10d4f3a).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
nireports/reportlets/mosaic.py	42.85%	4 Missing ⚠️
nireports/reportlets/utils.py	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #153      +/-   ##
==========================================
+ Coverage   64.78%   65.21%   +0.43%     
==========================================
  Files          23       25       +2     
  Lines        2644     2671      +27     
  Branches      367      420      +53     
==========================================
+ Hits         1713     1742      +29     
+ Misses        816      812       -4     
- Partials      115      117       +2     

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