Support vfwe_only in CBMAEstimator even when null_method isn't montecarlo

[tsalo]

Closes #385. Most of this code is excised from #659.

Changes proposed in this pull request:

• Allow vfwe_only, even when null_method == 'approximate'. This should run more quickly than the cluster-level approach.

[codecov]

Codecov Report

Merging #678 (848564d) into main (6cd751e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #678   +/-   ##
=======================================
  Coverage   85.28%   85.29%           
=======================================
  Files          40       40           
  Lines        4513     4516    +3     
=======================================
+ Hits         3849     3852    +3     
  Misses        664      664           

Impacted Files	Coverage Δ
nimare/meta/cbma/base.py	96.56% <100.00%> (+0.04%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6cd751e...848564d. Read the comment docs.