[REF] Move CBMA methods up a level

[tsalo]

Closes #220.

Changes proposed in this pull request:

• Remove the CBMA submodule to simplify the organization of the package.
• Add warning about likelihood-based estimators to associated docstrings.

[codecov]

Codecov Report

Merging #283 into master will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #283      +/-   ##
==========================================
- Coverage   74.49%   74.46%   -0.04%     
==========================================
  Files          39       38       -1     
  Lines        3717     3712       -5     
==========================================
- Hits         2769     2764       -5     
  Misses        948      948              

Impacted Files	Coverage Δ
nimare/base.py	66.85% <ø> (ø)
nimare/correct.py	91.02% <ø> (ø)
nimare/meta/ibma.py	98.59% <ø> (ø)
nimare/meta/peaks2maps.py	1.75% <ø> (ø)
nimare/decode/continuous.py	65.21% <100.00%> (ø)
nimare/meta/__init__.py	100.00% <100.00%> (ø)
nimare/meta/ale.py	93.24% <100.00%> (ø)
nimare/meta/kernel.py	79.87% <100.00%> (ø)
nimare/meta/mkda.py	92.36% <100.00%> (ø)
nimare/meta/utils.py	57.14% <100.00%> (ø)
... and 5 more

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