[MAINT] make indexed_gzip install optional

[jdkent]

Closes # .

removing indexed_gzip makes NiMARE a pure python package (and therefore usable by pyodide/jupyterlite, etc)

Changes proposed in this pull request:

• make indexed_gzip installable via [gzip]

[tsalo]

If indexed_gzip isn't necessary, it would be great to remove it entirely. It might be good to see if the original issue that led to us requiring indexed_gzip (#399) has been resolved (i.e., hopefully nibabel has dealt with this issue already).

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.17 🎉

Comparison is base (8fdc970) 88.74% compared to head (3bd7623) 88.92%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #791      +/-   ##
==========================================
+ Coverage   88.74%   88.92%   +0.17%     
==========================================
  Files          41       41              
  Lines        4825     4856      +31     
==========================================
+ Hits         4282     4318      +36     
+ Misses        543      538       -5     

Impacted Files	Coverage Δ
nimare/decode/base.py	93.18% <ø> (+2.07%)	⬆️
nimare/decode/continuous.py	96.87% <100.00%> (+2.08%)	⬆️

... and 2 files with indirect coverage changes

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[jdkent]

@tsalo do you know if there was a test written that covered the particular issue you referenced?

[jdkent]

looks like the ale workflow tests it, so this requirement is likely not necessary, but I prefer to keep it optional for one release before dropping it completely to see if people run into problems.