Convert permutation tests to functions

[tsalo]

MetaRegressionResult.permutation_test and CombinationTestResult.permutation_test both require access to the Result object, which is a bit much for NiMARE Estimators to hold onto between the fitting and correction steps. If we could convert the methods to functions, and possibly distinguish the null distribution creation step from the correction step, I think that could help NiMARE.

This stems from neurostuff/NiMARE#278.