Add DatasetSearcher class with search methods from Dataset

[tsalo]

Closes #669.

Changes proposed in this pull request:

• Create a nimare.dataset.DatasetSearcher class with all of the get methods originally in the Dataset class.
• Move several functions around to deal with circular import issues.
  • We probably need to enforce some kind of import rules for our modules to avoid this kind of situation.

[codecov]

Codecov Report

Merging #677 (bc67dbc) into main (d37c216) will increase coverage by 0.03%.
The diff coverage is 89.76%.

@@            Coverage Diff             @@
##             main     #677      +/-   ##
==========================================
+ Coverage   85.32%   85.35%   +0.03%     
==========================================
  Files          41       41              
  Lines        4503     4526      +23     
==========================================
+ Hits         3842     3863      +21     
- Misses        661      663       +2     

Impacted Files	Coverage Δ
nimare/utils.py	94.72% <ø> (+0.03%)	⬆️
nimare/workflows/macm.py	25.00% <33.33%> (-0.72%)	⬇️
nimare/meta/kernel.py	80.36% <75.00%> (+0.24%)	⬆️
nimare/dataset.py	89.76% <89.30%> (+0.04%)	⬆️
nimare/meta/utils.py	55.06% <92.30%> (+2.26%)	⬆️
nimare/annotate/cogat.py	91.35% <100.00%> (ø)
nimare/base.py	85.96% <100.00%> (+1.14%)	⬆️
nimare/decode/base.py	91.48% <100.00%> (+0.37%)	⬆️
nimare/decode/continuous.py	94.84% <100.00%> (+0.16%)	⬆️
nimare/extract/utils.py	42.22% <100.00%> (+3.63%)	⬆️
... and 7 more

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