TYP: Annotate loadsave and image header classes

[effigies]

nb.load() and (to some extent) nb.save() are probably the most used functions. They should have some signatures, though the usefulness is bounded as the type checker can't infer the resulting type from the filename.

While testing this on a code base I'm writing to be type-checked from the start, I found that everything declared itself as having a FileBasedHeader, which is not very useful. I can't come up with something type-variable-ish so that we can deduplicate these and rely on the header_class, but oh well.

[codecov]

Codecov Report

Patch coverage: 94.28% and no project coverage change.

Comparison is base (40e31e8) 92.16% compared to head (9f189c6) 92.16%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1213   +/-   ##
=======================================
  Coverage   92.16%   92.16%           
=======================================
  Files          98       98           
  Lines       12366    12372    +6     
  Branches     2545     2543    -2     
=======================================
+ Hits        11397    11403    +6     
  Misses        646      646           
  Partials      323      323           

Impacted Files	Coverage Δ
nibabel/nifti1.py	92.76% <50.00%> (ø)
nibabel/analyze.py	98.59% <100.00%> (ø)
nibabel/brikhead.py	97.51% <100.00%> (+0.01%)	⬆️
nibabel/cifti2/cifti2.py	96.86% <100.00%> (+<0.01%)	⬆️
nibabel/ecat.py	88.21% <100.00%> (ø)
nibabel/filebasedimages.py	97.29% <100.00%> (-0.04%)	⬇️
nibabel/freesurfer/mghformat.py	95.08% <100.00%> (+0.02%)	⬆️
nibabel/imageclasses.py	100.00% <100.00%> (ø)
nibabel/loadsave.py	70.53% <100.00%> (-0.77%)	⬇️
nibabel/minc1.py	90.36% <100.00%> (ø)
... and 5 more

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