simplify datasets() method

[afeld]

Also add an all_datasets() method.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.57%. Comparing base (176b331) to head (f1e6dcd).
Report is 6 commits behind head on v3.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##               v3     #101      +/-   ##
==========================================
+ Coverage   89.23%   93.57%   +4.33%     
==========================================
  Files           8        8              
  Lines         381      389       +8     
  Branches       50       43       -7     
==========================================
+ Hits          340      364      +24     
+ Misses         27       15      -12     
+ Partials       14       10       -4     

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