Generalize needs_dataset decorator to non-Ion objects

[wtbarnes]

Fixes #315

[codecov]

Codecov Report

Attention: Patch coverage is 95.91837% with 2 lines in your changes missing coverage. Please review.

Project coverage is 93.14%. Comparing base (7256ed3) to head (25a4f37).
Report is 19 commits behind head on main.

Files with missing lines	Patch %	Lines
fiasco/util/tests/test_decorator.py	95.65%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #316      +/-   ##
==========================================
+ Coverage   92.65%   93.14%   +0.48%     
==========================================
  Files          37       38       +1     
  Lines        3119     3165      +46     
==========================================
+ Hits         2890     2948      +58     
+ Misses        229      217      -12     

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[jwreep]

That was fast! Thanks!