Add `nimads` module with implementations of NIMADS objects

[tsalo]

This will probably never be merged, so I will say that it references #218 instead of closes it. I just wanted to put together a skeleton of the nimads module, so that we can determine what's necessary vs. optional and what might be difficult to implement in NiMARE.

[codecov[bot]]

Codecov Report

Base: 88.64% // Head: 84.67% // Decreases project coverage by -3.97% :warning:

Coverage data is based on head (910037e) compared to base (282f197). Patch coverage: 68.02% of modified lines in pull request are covered.

:exclamation: Current head 910037e differs from pull request most recent head 8c4d359. Consider uploading reports for the commit 8c4d359 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #628      +/-   ##
==========================================
- Coverage   88.64%   84.67%   -3.97%     
==========================================
  Files          38       43       +5     
  Lines        4324     4693     +369     
==========================================
+ Hits         3833     3974     +141     
- Misses        491      719     +228     

Impacted Files	Coverage Δ
nimare/nimads.py	64.22% <64.22%> (ø)
nimare/workflows/neurosynth_compose.py	69.23% <69.23%> (ø)
nimare/io.py	95.37% <100.00%> (+0.22%)	:arrow_up:
nimare/workflows/__init__.py	100.00% <100.00%> (ø)
nimare/meta/utils.py	54.29% <0.00%> (-41.44%)	:arrow_down:
nimare/meta/kernel.py	80.12% <0.00%> (-15.67%)	:arrow_down:
nimare/extract/extract.py	42.97% <0.00%> (-4.90%)	:arrow_down:
nimare/meta/cbma/ale.py	94.92% <0.00%> (-1.70%)	:arrow_down:
nimare/meta/cbma/mkda.py	97.08% <0.00%> (-1.68%)	:arrow_down:
nimare/meta/cbma/base.py	95.63% <0.00%> (-0.39%)	:arrow_down:
... and 23 more

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[jdkent]

ref neurostuff/neurostore#269

[tsalo]

@jdkent would you be willing to link any issues this should close?